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MG MGB Technical - Engine shaking
| Hi All, My first time on this BBS. I have a 1967 or there abouts mgb roadster, that was supposedly restored to 95% concourse standard. 10 months on and I'm still trying to complete it. After having solved a myriad of problems I am now stuck with this one, at about 1500 RPM the engine shakes, with sideways displacement of about 1 1/2 inches (~40mm) , this is standing still, ie not driving. Higher revs stop the large shaking but there is still a vibration apparent, felt through the body of the car. Lower revs, ~ 1000 rpm the engine runs reasonably smooth, about what you would expect at a fast idle. Details Engine is 5 bearing, cast November 1965. New coil New 123 distributor (old distributor does the same shaking) New plugs and leads New jets and needles in the carbys New float valves and level set correctly New ECCO fuel pump and pressure regulator, set to 2 psi Fuel filter fitted I've had the head off and found that the engine has been recently rebored to 60 thou oversize. Found that the PCV valve was missing some of its works, removed it and connected the ports on the carbys for crankcase ventilation. Resealed all ports on the inlet manifold. I currently have the timing cover off to check that the camshaft is correctly timed, it is. The only things I can think of that might explain this shaking is a very hot camshaft or massive unbalance in the engine. Any suggestions very appreciated. Herb |
| H J Adler |
| Harmonic balancer pulley might be disintegrating. Fan lost a blade or loose blade. Worn or loose engine or gearbox mounts. What are the compression readings? |
| Art Pearse |
| Sounds like a cylinder or two are not firing. Maybe fuel starvation? Try setting the fuel pump to 3-4 psi. Also, test the spark plug(s) and lead(s) to verify them to be working. |
| Daniel Wong |
| Thanks for your replies. Art Harmonic balancer and engine / gearbox mounts OK. Compression 170 psi all cylinders. Daniel My thoughts earlier that a cylinder is not firing, tested with strobe and all leads indicate spark energy. I have found that my Xenon strobe will not trigger if there is no spark, ie the plug is shorted. Had this earlier before finding the PCV problem. Plugs are now uniformly black, but firing. I know too rich, but can't seem to get the proper mix, which is why I have replaced the major fuel items. Engine is not "working" ie never has had a load on it, just running whilst car is stationary and I think that actually driving would burn off the carbon. Also the problem occurs immediately after cleaning the plugs and replacing them. Not much time to get fouled. Fuel starvation seems not to be a problem because the engine will happily run to 3-4000 RPM and I'm not game to run it beyond that. Also fuel starvation would create massive backfiring as the mixture gets too weak. Herb |
| H J Adler |
| Check the compression? Maybe built-up carbon is hanging open a valve? Try getting the over-rich mixture to proper as this can contribute to misfires. |
| Daniel Wong |
| Grossly over-rich will cause engine shaking due to the rhythmic effects, but a rev dependant shake to that level does indicate imbalance. I'd still want to correct the mixture before digging any deeper into the engine - with black plugs you can't tell whether they era firing or not, and that should only be done when you are sure ignition is correct - you could have plug leads in the wrong order, like two reversed, the order should be 1 3 4 2 *anti*-clockwise, once you have determined which is No.1 on the cap by seeing where the rotor is pointing at the top of the compression stroke, which is determined by turning the engine with your thumb over No.1 plug hole, it will be lifted off during the compression stroke. A strobe only shows that HT is reaching the plug, not what is happening inside the cylinder, and a plug lying outside the cylinder won't show what is happening under compression. The only way to check the plugs and leads is to disconnect each lead in turn and make sure that running obviously gets rougher, equally in each case. Any lead that causes no change in engine running isn't firing a mixture. Whether that is down to the plug or lack of mixture would remain to be seen. In itself a broken engine mount shouldn't cause this if the engine is balanced, I've seen an engine started and run just sitting on a bench, and when a mount was broken on my V8 all that happened was that side lifted up under acceleration. |
| Paul Hunt |
| Hi Herb, I would certainly get your mixture right asap. I use a "Colourtune", which I have had for more years than I can remember, plus a "carbalancer". With these two pieces of equipment I find it very easy to get the carbs set up nicely. Once you've got a nice blue bunsen flame on each cylinder, hopefully your engine shake will have disappeared. You need to fit the "colourtune" to a front cylinder to set the front carb, and then the "colourtune" in a rear cylinder to set the rear carb.. I also bought a auto multimeter to set an accurate tickover as the tachos in the car are not that accurate. Do follow the manual for getting/maintaining the engine at operating temp. during your adjustments. Good luck with your quest. HTH willyphixitt |
| W A Nixson |
| Hi Herb, I rebuilt an engine more than a few years ago and put the dizzy back in 180 degrees out, ( it was easy to do ) It fired up and jumped about. Just looking at the cap and leads will fool you. Check the the rotor arm is pointing at the right lead. |
| c cummins |
| Herb, ColorTunes don't seem to work so well in the United States anymore with our oxygenated fuel, not sure about Australia. To see if the carburetors are set correctly, remove the dampeners, take off the suction chambers and pistons, being careful to not mix up the parts. Each should have a spring. Looking down from the top, lower the jets (choke) until fuel is visible. The fuel should be .160 (.120 to .200) below the bridge of the carburetor inlet. Playing with the jets and watching the fuel will get you level of the fuel, meausure with the stick end of a dial caliper down with the jets at fuel level. Raise the jets full up against the stops. Adjust them so they are .070 below the bridge of the carburetor inlet. This is your starting point. Reassemble the carburetors. If you want you can do a drop test by covering the vent hole and see how long it takes for the piston to drop out of the suction chamber. They should be uniform. Balance each carburetor at 1500 rpm if you haven't done so, and then idle back down to 800 RPM. Move both jet adjusting nuts the same (one flat at a time up or down) until max idle is achieved. Then use the lifting pins to check for the proper mixture, lowering the idle to 800 if needed. This will put you well into the proper range. warmly, dave |
| Dave Braun |
| Hi All, Again thanks for all the replies. Daniel I checked the compression and it was 170 psi, 10 psi over spec. This made me remove the head to see if excessive carbon build up was the cause. That's when I found that the engine had been overbored by 60 thou. All carbon deposits were a black layer which came away with just wiping with a rag. I must admit that as yet I haven't checked whether the valves are operating correctly, will do next time. The rich mixture has been a problem from day one. All the fuel changes have been done to try and correct this.I'm at less than one turn down from the bridge, having started at 1 1/2 turns. Paul I will try and get the mixture correct when I get the engine running again. Hopefully there is some tuning that is wrong, since I don't relish having to go through dismantling the engine to try and find the imbalance. By any chance do you know of a way to check the balance without dismembering the engine? My strobe is electronic with a clip on sensor and I have found that if a plug is not firing ( shorted or what ever ) the strobe will not trigger, probably not enough of a voltage rise. The firing order. I dream of it, having checked it umpteen times. Because the plug leads have huge rubber shrouds over them it's impossible to simply short out a plug to check. Disconnecting is a worry as it may overstress the ignition system by operating into an open circuit. I plan to make up some "extension" leads so that I can short them out one by one. I believe timing as such is correct as the advance as seen on the harmonic balancer with the strobe, at various engine speeds corresponds roughly with published curves. willyphixit My tacho is spot on since I have calibrated it recently. I always perform any tuning exercises at operating temps. c cummins Dizzy is correctly installed, checked many times. Dave I have set the fuel level to 3/8" below the bridge, as suggested by SU themselves. My starting point for setting the mixture is 1 1/2 turns down from the jets being level with the bridge. I'm not sure what you mean by the "drop test" and the "vent hole". Since my problem is at 1500 RPM, I have balanced the carbys with a flow meter at about 1000 RPM. Pushing the pins up individualy on each carby does not seem to make any difference, however, pushing both up at the same time does increase the RPM, indicating a rich mixture. Herb |
| H J Adler |
| Herb - I don't know of a way to check balance without dismantling. In my experience if a plug is not firing but HT is reaching it a clip-on timing light will flash. If it doesn't flash then HT is not reaching it which usually means the rotor or cap is breaking down (if the coil lead is flashing correctly, that is). This is regardless of whether the plug is open-circuit or short-circuit, and I have proved this is the case in tests as well as in diagnosing faulty ignition. I have a gunson's light, others may vary, but I have seen exactly the same effects with the old-style series-connected timing lights as well. |
| Paul Hunt |
| In a recent post there was a case of a bad dizzy cam which caused different timing on individual cylinders. Could this be the case, caused by shaft / bush wear which "straightened out" at high rpm? |
| Art Pearse |
| Hi Thanks Paul. I was pretty sure that to check balance you had to dis assemble the engine. Note your comment about strobes. I will attempt to get the mixture right once the engine is running again. Art, Brand new 123 electronic dizzy. Herb |
| H J Adler |
| Herb, I seem to recall that there are two main jet sizes available for the SU, and that they are physically interchangeable. Any chance you have the large ones and need the small ones? You said you removed the PCV valve. Can you do that on your set up without introducing too much crankcase air to the intake? You said you removed the timing cover to check cam timing. Did you check it with a dial indicator and degree wheel or with the dots on the chain wheels? If the cam is not stock, the dots are likely meaningless. If the cam is not stock, and you don’t have the specs, you would have to guess at the setting if using the degree wheel and dial indicator. Charley |
| C R Huff |
| This problem cannot be anything to do with carbs. It is easy to check if a 4 cyl is running on all cylinders - just listen to the exhaust, regular or not? What could have happened to alter the balance? How about a bit of piston skirt has broken off? Unlikely I know, but the answer is probably unlikely! |
| Art Pearse |
| Hi, Charlie You are right, there are two jet sizes, 0.09" and 0.1", however I have the correct 0.09 ones. About 1968 the carbys had port inserted between the piston and throttle disk which gives a constant low depression and is used as the crankcase ventilation port. My carbys are of this type so I discarded the PCV valve and ran hoses to the ports on the carbys. See link below. http://www.mgb-stuff.org.uk/enginetext.htm#breathingp I only checked the cam as to the two dimples. Considering all the errors I have found in the car it wouldn't have surprised me if the cam were out a tooth or two. Art I think the engine had not been run since a rebuild. Brand new pistons. My nightmares on balance are things like the pistons / rods in 180 deg out, ( by the by the pistons have a notch on the edge, which is facing forward. I assume that these are the "front" indicators.)flywheel on wrongly or not original and equally nasty things. Herb |
| H J Adler |
| Herb, I bought a GT with a "fresh rebuild" which did have new parts, but put together very poorly. It had a custom grind cam that was way out of time. As a result, it only had 105 psi compression. I moved it two teeth (just a flat out guess) and got it up to 145 psi. Then, for that and other reasons, I tore it down and rebuilt it myself. So, based on your compression figures and my experience, it seems likely that you do not have a cam timing problem large enough to cause your problem. Regarding the vacuum port between the piston and throttle disc, that sounds like the port for the vacuum advance to me. I thought around 68/69 they still used the Smiths mushroom PCV valve on the manifold, and the direct connection between carbs and crankcase didn't occur until the HIF carbs. My car is made up from lots of parts from here and there, so my knowledge is a bit scattered as a result. Charley |
| C R Huff |
| Herb - rods or pistons in 180 deg out? Pistons are possible but surely would not cause out of balance, aren't they symmetrical about the pin axis? Rods - I don't think you could assemble this way due to offset, but if you could, should not cause out of balance. Sounds like you did not rebuild it yourself and there is something seriously mechanically wrong. I would strip it down and check everything. |
| Art Pearse |
| The port for crankcase breathing is between the butterfly and the piston as Herb says, on both carbs and the Zenith from Oct 68, and quite large at about 1/4". An earlier car could well have later carbs. The port for vacuum advance was above the butterfly on the rear carb only, much smaller, and from inception until 1971 for North America and Oct 76 for the UK (by which time they were HIFs). The two ports give quite different signals. |
| Paul Hunt |
| Hi All, Thanks for all replies and comments. I've just spent another weekend on it, in 34 deg C heat (94 deg F). My original post was very subjective about the extent of the shaking. Whne sitting down and visualising the sideways movement it became greater than it really is. Objectively its only about 3/4", still very bad. After reassembling the tiiming cover etc, I made some observations, and decided that balance was not the problem. I reved the engine and then turned the ignition off. It died smoothly, until the last rev or so when it did the usual last minute shudder. No shaking. Front cylinders were an acceptable colour, back ones black. Cleaned them and played with the carbys till the back one had a momentary increase in revs and then started to die. Cleaned the plugs again and ran the engine for a few minutes. Rear plugs black again. Really p...ed off now. Undid the fuel tube from the float chamber and inserted clear plastic tube, turn on the ignition so the fuel pump would run and the fuel came up too high in the tube. Ie the fuel level in the float chamber is too high, probably causing the rich mixture. I'm on my third set of SU needles and seats, the ones with the rubber cone on the end. They seem to be crap. Have ordered Grosse ones, hopefully will cure my problems. Herb |
| H J Adler |
| The new Grosse jets will not cure any problem. They will create new ones. they are not up to the original standards. Use the viton tipped needles and make certain that you have no more than about 2-4 lbs pressure. check the float level by removing the covers and pistons. With the key on the gas level should be about .060 below the bridge. Higher than that and you cannot adjust the jets. sandy |
| SANDY SANDERS |
| Have you checked to determined whether if the carbs (not the engine, itself) are balanced? |
| Daniel Wong |
| Hi, Sandy, I've tried three sets of the viton tipped needles, all NBG. The Grosse ones can't be any worse. My fuel pressure is regulated to 2 psi. Daniel, The carbs have been balance with a flow meter. Herb |
| H J Adler |
| You say the fuel level in your test tube came up too high. It can only rise higher than the level in the float chamber if the float chamber vent is blocked. If the level in the float chamber *and* the test tube rises higher than it should, then a) you should be getting fuel continuously coming out of the vent i.e. an overflow, and it indicates the float valve is bad. If the level is higher than you think it should be but is still lower then the level of the vent so isn't overflowing, then float height could be the problem. But it could also point to float valves again if it needs the float to be pressed up against the valve before the valve stops. You can certainly try Grosse 'jets', but three float valves can't really all be bad (unless they are from the same faulty batch of course). If the problems are only with the rear carb then there must be something wrong with that carb, I'd swap the float valves over if the front one seems to be good and the rear ones repeatedly bad. |
| Paul Hunt |
| Hi Paul, I agree. My original venture into the float valves was because of fuel spurting out of the overflows, hence the first set of replacement valves. Then when there was still overflow, the pressure regulator. After this I set up a test bed with a jar, coloured water and plastic tubing to see what actually is happening with the float valves. The ones that came in the carbys looked new, ie no sign of any marks on the rubber tip. The second new set, identical to those fitted to the carbys, that came with the jet kits and a separate set purchased on their own all had the rubber tip. The purchased ones were different in that they had a spring inside the actuator. On the test set up I mounted a float cover on a jar so that I could see what was happening, with a piece of clear plastic tubing strapped to the side of a ladder to create a bit of a pressure head. The floats didn't leak and their shafts were replaced, as there was considerable wear on them. The first valve I tested let the float be fully immersed in water and still didn't cut off the flow. Tested the float on its own to see what its bouyancy was, only about 1/3 in the water. So this valve didn't work Ditto for the second set that came with the jets, still leaking with only about a foot of water head, ie a pressure of less than 1/2 psi. The ones with the spring shut off the flow, though a very slight residual weep, about 1" in ten minutes, on my water head. So I installed these and set the gap to 7/16", the max you can do, which gave a fuel level about 3/8" below the bridge of the carbys, this being the recommended level from SU themselves. Now, as I mentioned in my last post, the level in the rear carby is somewhere around the bridge height, not enough to overflow but still higher than it should be. Herb |
| H J Adler |
| I'd still swap float valves over, and floats and hinge pins as these don't seem to be handed, on HSs at least, which I'm assuming you have. If the problem stays in the rear carb then there can only be something odd with the float chamber or lid itself. The chambers are interchangeable, and although the Parts Catalogue doesn't show or list the lid for the rear carb for some reason, it probably is handed, although I'm sure you could jury-rig them on the other sides to test. One problem like this I seem to recall was an incorrect float, which although it 'floated' as it should when tested outside the carb was fouling something inside the float chamber hence flooding. |
| Paul Hunt |
| "The new Grosse jets will not cure any problem. They will create new ones" Me thinks not. Replace Viton needles that leaked constantly (overflow). Grosse jets have worked like a champ in my daily driven '63 for the last 6 months. If your level is too high, either the flots are leaking or the jets don't seal. I assume you have checked to see if the floats have any liquid in them. After that, try the grosse jets and see. You could run the moter without fuel pressure, if the shaking goes away after 10 or so secs, your fuel level is the problem. |
| GLP Pelech |
| Have you checked that the washer with the internal grooves is installed on the underside of the overflow pipe with the aluminum washer on the top. also check that the overflow pipe is clear. sometimes they become blocked. This tube being blocked combined with a leaking float valve will make that carburettor run extremely rich. Sandy |
| SANDY SANDERS |
| Just one other thing that can make the engine run rough is that the balance holes in the front face of the carbs have been blocked off by an incorrectly fitted gasket or air filter backplate. |
| Chris at Octarine Services |
| Ironically a pal's 78 has gaskets between the carbs and air-filter back-plates with two sets of holes so they can't be fitted incorrectly. "Very neat", I thought, but still managed to get the backplates upside down! Seemed to run as well as normally, but it did fail slightly high emissions. A blocked vent/overflow will stop the float rising to operate the valve and so the pump will continually operate pushing fuel up the jet and down the inlet manifold. |
| Paul Hunt |
| One possibility for a leaking jet is that the jet assembly has a bad thread into the bowl cap, and it is leaking round the jet, no through it. |
| Art Pearse |
| Hi All, MY GROSSE VALVES ARRIVED TODAY, Thursday. Will install them on the WE. Paul, I appreciate your comments about swapping the floats etc, however, at various times both carbys have exhibited overflow problems. All my tests were actually done with the front carby's cover. I will see how the Grosse valves go and if they also don't work will try your suggestions. Sandy Now I'm really confused! What washer with grooves under the overflow, etc.? Do you mean the washers under the float valve seat to adjust fuel level? The overflow tubes are definitely not blocked, fuel has gushed out of both at various times. Chris Breather holes definitely not blocked, as no filters attached yet. Art I think you are thinking of much earlier SUs, mine are HS4s, where the jet is connected directly to the float bowl via a tube. No way that fuel could leak past anything to cause a rich mixture. Will post results of fitting the Grosse valves. Herb |
| H J Adler |
| Herb. The washers are the ones that go between the overflow tubes and the float valve cover. There are two of them. those washers are round on the outside with recesses cut on the internal hole. They go on first followed by the overflow tube then an aluminum washer all held on by the top hollow bolt. If you transpose these two washers then you have sealed the outlet to the overflow tubes. Sandy |
| SANDY SANDERS |
| I'm thoroughly confused by Sandy's suggestion as well. AFAIK the overflow is simply a hole in the float chamber lid that goes to one of the brass ports that sticks out of the top of the lid. There are no such washers or bolts shown in the Parts Catalogue. The only hollow bolt I can think of is on the fuel pump banjo unions. But I think Art is referring to the float valves when he says 'jets' (and I could never understand why Grosse call their items 'jets' either). If the valve or washers isn't sealing to the float lid then fuel could be leaking past, which I why I suggested swapping lids as well as other parts. But Herb saying it has happened to both, and through multiple float valve changes, it really points to excessive fuel pressure. You say you have a fuel regulator, and a non-standard fuel pump. Have you set the regulator to a 2psi mark on the regulator? Or actually measured the pressure? If the latter have you compared that with anything else to confirm the reading? Other than that it can only be incorrect parts in both carbs, and it can only be the float valves if all are from the same batch and have the same fault - or are the wrong ones. |
| Paul Hunt |
| I think Sandy is talking about an earlier version that had a single central hollow bolt holding the float cover on. It also held the overflow tube. I think this was before the models that had three screws holding the float cover. Charley |
| C R Huff |
| Yes, I meant the float valves. Same query - is it leaking around? If so, then some sealant on the thread might fix it. |
| Art Pearse |
| Hi, I think I have a picture of the type of SU that Sandy talks about. No idea of its vintage, but suspect that it is before 1958, as my "58 Austin Lancer had the type with the three screws. See picture. Herb  |
| H J Adler |
| Yes, Sandy is talking about the H4 type (MGA). All MGB are HS4 until HIF came along. You may all want to refer to Flip Bruhl's thread "One eyed Frogeye", in which after much consternation he discovered that SU knew what they were about when they used spring loaded float valves. I've never been able to understand the insistence on either "Viton tipped" or "Grosse Jet", when the correct SU valves give no trouble ever. Given your vibration problem, this seems a good move. FRM |
| FR Millmore |
| Hi All, Art Threads have been sealed on float valves. FRM The viton tipped valves seem to be genuine SU. The jet assemblies I purchased came with a full set of repair parts, including needle valves, which were the same as those in the carbys, ie viton tipped. Over the last 40 years I've become aware of at least 4 different types of float valves. The earliest were solid brass, then came plastic (unknown kind) with metal tip and spring loaded. I used all of theses with no troubles at all. Now they are brass with viton tip, from SU, and apparently third party ones of aluminium with viton tip and spring loaded. Paul I had thought of leaking around the threads and had sealed them. With all my testing I used a home made water gauge, the only thing I could think of to easily measure such low pressures. I calculated that for the highest specd SU pump of 3.5 psi, the water height was about 7.75 feet. I stopped the pump at 10 ft, about 6 psi. That's when I bought the pressure regulator. I assume that the pressure regulator is pretty spot on. Also when resting the viton tipped valves the best would still weep with only about 1 ft of water head, a pressure less than 1/2 psi. Now I have fitted the Grosse valves. Fuel level is perfect and steady. I did both static and dynamic, ie with engine running, tests. With the dynamic test there was a slight dip in level when accelerating, but this quickly came back to the mark. On decelerating the opposite effect, a short increase in level before returning to the mark. See image of the test set up I made with plastic tubing and T piece. When the carbys are empty the pump runs and then stops and stays stopped, no occassional tick, as previously. In fact I ran out of petrol (gas) and then couldn't hear the pump tick. Swear curse, now I've got pump problems too. Messed around checking voltage etc. The pump was OK, it just found enough fuel to pressurise the line and stopped. I am now happy that fuel feed problems are cured. Unfortunately the engine still shakes. See next post about further tests. Herb  |
| H J Adler |
| Hi Again, After setting the carbys for mixture, the rear one (feeding the problem cylinders 3 & 4) was set so that there was a slight acceleration then die, which is the supposed correct setting. I did the same adjustments to the front carby as I did to the rear one. Plugs still black, engine still shakes. I now did some tests on the electrical side of things. First I made up some ignition lead extenders, see image, so that I could selectively short out plugs. The result of this was a bit surprising, as shorting 1 & 2 both caused theengine to run rough and sloew slightly, expected result. However, shorting 3 & 4 changed the exhaust note, didn't noticably change engine revs, but, and this was the strange thing, the engine ran smoother. Plenty of spark energy at the plugs, as I found out when my earthing strap slipped off the body connection. I copped it through insulated crocodile clip and insulated pliers. I was holding the clip with the insulated pliers, to be on the safe side. HA! I checked the timing as well, and numbers 1&4 were both accurately on 10 deg BTDC. As best as I could tell 2&3 were also OK, but had to eyeball measure as they fire 180 deg away from timing marks. Now I'm totally stumped: Front carby shows no affect if the test pin is pushed up to raise the piston, rear carby shows behaviour consistent with correct fuel mixture. Front carby's cylinders show expected results when shorting out the plugs, rear carby's cylinders show virtually the opposite to what one would expect, when shorting out the plugs. The colour of the plugs still indicate that they are running rich! I am wondering if there is some strange pre ignition happening, causing my engine to shake so. Note that the shakes happen straight after starting from cold, so there shouldn't be enough time for a hot spot to develop. Oh I almost forgot to mention that the float bowl of the rear carby gets quite hot after running the engine for a while. The heat shield is new and the insulation pieces intact. But again the problems are apparent from cold. Herb  |
| H J Adler |
| Are you 100% sure the firing order is correct? 1 3 4 2 going anticlockwise on the dissy cap. If the engine runs smoother with 3 or 4 shorted, it points to those cyls firing at the wrong time. |
| Chris at Octarine Services |
| Herb, I thought that if lifting the pin made the engine die, it meant it was too lean. But, you have black plugs on 3 & 4. I thought that too rich would make the engine speed up and stay sped up when lifting the pin. Then, your front carb shows no change when lifting the pin. So, this may seem a bit far-fetched, but I did read it in some publication, and I did have it work for me at least once, maybe on a Volvo with SUs. What I read was that response to lifting the pin indicates the problem, but that the problem is with the OTHER carburetor. The theory kind of fits your pin lift results. If you are desperate, you might try it. Charley |
| C R Huff |
| Try changing the plugs in 3&4. They could be shorting the spark. |
| Art Pearse |
| Herb- The spring loaded/metal tip plastic ones are the SU ones I've always used - AUD9096. I recently got some in a kit from Moss, think it was a Burlen kit, in which they were numbered AUD9095. I agree with Chris. Might also be a defective cap or bad wires causing the same crossfire. Charley's story is true, when carbs are very far off - the effects frequently "read" backwards carb to carb. I just start off at standard basic settings in that case. Have you checked the piston springs? It is common for people to stretch them - I've seen some two feet long! Or, they switch them around from other cars with different springs. Too stiff a spring will make it run rich, leaving you no adjustment, or you get the idle adjustment correct but it's way rich above idle. leading to fouled plugs. Also, do the drop test timing on the pistons. If some idiot has sand papered the vac chambers and/or pistons, you are SOL. FRM |
| FR Millmore |
| Hi All, Thanks for all your feedback and thoughts. Charley You are partially correct, too rich it speeds up, too lean it dies, however, spot on it briefly speeds up then dies. Chris, Definitely 150% sure that the firing order is correct. The first and last thing I check every time. Art, The second set of plugs, no change. With the number of times I've cleaned the plugs ( I have one of those old spark plug cleaners, a miniature sand blaster, and yes I blow the sand out of the plugs after cleaning them. FRM Brand new ignition wires and 123 dizzy,set to curve 5, which is identical to the standard B curve. The symptoms also present with the original Lucas 25D dizzy, which by some quirk in the past is actually from a Mk1 Hillman Imp. I've checked the springs for length, both the same and correct for the standard B springs, as is the compression with weights, so assume that they are the correct springs. I would have thought that a too stiff a spring would cause lean running, by keeping the needle pressed down into the jet too much. I have started the fuel mix adjustments for both carbys at 1 1/2 turns down from jet level with the bridge. Any adjustment I did on one, the rear one, as recommended in some tune up guide, I also did to the front one, ie one flat up on the rear, then also one flat up on the front. Carby throttles were synched with a flowmeter. I haven't specifically checked the pistons and chambers, but whilst handling them I didn't notice any abrasion marks, good thought though, will check next time. When I fired it up this morning I couldn't get it to fire, just coughed and wheezed and stalled. Cleaned the plugs, black again, and then tried starting and it did fire, kept running, but again with lots of backfiring, coughing etc. Could be because I don't have the choke connected. Ran OK, except for my problem, yesterday. I've totally run out of ideas, and any more forthcoming will be greatfully accepted. Herb |
| H J Adler |
| Herb- Well if you keep eliminating stuff, what will we guess at?! "I would have thought that a too stiff a spring would cause lean running, by keeping the needle pressed down into the jet too much." No. The airflow speed increases sufficiently that more fuel is pulled out of the jet, even though the needle is blocking it a bit more. That's why the damper causes a rich mixture for acceleration. Do the piston drop timing test. I take it the air filters are off while you do all this? Are the carb needles correctly marked and have you measured them to compare to specs? Are they set shoulder flush with the piston face? You might gauge the new jets too - mistakes happen! You checked the cam by the marks? Not reliable, check it with an indicator setup, or at least by the book quick check method. Since you don't know what the cam is, you might want to do a cam timing diagram plot, to find out exactly what you have. You might try using your timing strobe to watch the valve gear when the shaking is going on. It only gives you 4 "windows" if you change it around on the plug leads, but it could show any irregularities. Like valves hanging in 3 & 4 and lighting off the other cylinder. FRM |
| FR Millmore |
| Hi Herb, I've been following this thread, but not contributed before as others have already posted the same questions/ideas that I would have put. One other thing to eliminate is your exhaust manifold. If there is a blockage/partial blockage then you will certainly encounter wierd problems. As you have spent so much time going over and over the obvious, it's niow time to do some lateral thinking. HTH and good luck. willyphixitt |
| W A Nixson |
| Herb, I thought the spot on indication for pin lifting was briefly speeding up, and then continuing to run a little rough. And, that continuing to run faster was too rich, and that dying was too lean. Are your jets centered so that the piston doesn't hang up at the bottom? Once someone told me that, for competition, blocking the balance tube would be an improvement. I have a Sprite that I built for autocross, which has B carbs and a modified B manifold. When I blocked the balance tube, it felt like it was running on 2 or 3 cylinders. It probably was. Unlikely to be your problem, but might be worth a check to see that someone didn't block it. FRM, Thanks for the confirmation on the opposite carb theory. I didn't think I had just made that up. Charley |
| C R Huff |
| My experience is that when correct it briefly speeds up then settles back, not dies (although maybe I don't leave it raised for long enough after that!). Rich stays speeded up, weak dies straight away. The difference in reaction to shorting different plugs definitely shows there are different combustion characteristics across the four. I'm with Chris on firing order, it would explain both the shaking and the fact that two of the cylinders change the exhaust note and shaking without changing the speed. Just as a laugh swap those two over and see what happens. But then again, it could still be problems with the rear carb. To check that you really need a new set of plugs run for short periods of time between being examined to see how they change. |
| Paul Hunt |
| Hi All. Again thanks for your thoughts, and I will check them out. Whilst lying in bed about 5 am I was going over the whole setup and mentally checking off everything I've done and tried. I was thinking about pre ignition and what could cause this. Then I thought that there is only one component that I haven't checked, changed or tweaked - the fuel. I am using unleaded premium ( 95 or 97 , not sure ) Before delving into the guts of the motor more I will try an anti knock / octane booster additive, until I use up the fuel in the tank. If it solves my problem then will need to use super hi premium or something. Herb |
| H J Adler |
| Have you pulled the valve cover and watched the rocker arms in action? I had a similar experience and checked everything thay I could think of to find the problem. After removing the valve cover, I saw that two rocker arms were barely moving due to a faulty camshaft that had two lobes scraped almost flat. Worth a look. RAY |
| rjm RAY |
| Herb - You are not getting detonation from low octane sitting in the shop, it only happens under load, assuming you haven't got kerosene/paraffin in it. FRM |
| FR Millmore |
| Could you upload some detailed pictures of the engine and its ancilliaries - an experienced eye might spot something. It's a bit like fishing in the dark without! |
| Chris at Octarine Services |
| Herb, On your petrol issue. My 69 18GG will not run smoothly on anything less than 98RON - I am using Castrol, have also used Shell in the past. Both standard and premium unleaded (I think around 91 and 95 RON at best respectively here) cause irregular running and serious run-on. At 60 thou over in the cylinders, somebody has had the opportunity to beef up the cam - and probably has done so. We don't know what head chamber volumes you have sitting on top of those cylinders. I'd be inclined to drain the tank and try a run on the 98Ron. In the event that there is no change in the shaking/running smoothness issues, it will not cause any additional problem just from the higher RON value. If it produces a change - removes or even just reduces the severity of the symptoms - it will point to a direction for further investigation. Regards Roger |
| Roger T |
| If you were getting shaking due to pre-ignition when idling either the timing is hugely over-advanced or there is something else very wrong with the engine. Easy enough to check the former, and just see what happens if you back it off anyway. But fuel would surely affect all cylinders equally, on its own it wouldn't cause the differences you noted when shrting 1 and 2 plugs compared to 3 and 4. |
| Paul Hunt |
| "If it produces a change - removes or even just reduces the severity of the symptoms..." I still think, in the circumstances I mentioned, a tank of 98RON is worth experimenting with. Heavens, Herb has just about exhausted options short of rebuilding the engine! Regards Roger |
| Roger T |
| I had this problem on a later MGB with a brake booster. The engine was ropugh at idle and appeared to run on two cylinders. It was a leak in the booster. Plugged the line with a temporary pencil and all the idle problems went away. Sandy |
| SANDY SANDERS |
| Extending from Paul's point that fuel would be expected to affect all cylinders equally, I wonder if there may be some physical interference to prevent that. Herb, have you checked that the butterflys are swinging equally with throttle movement, and are you sure that your resealing of the inlet manifold ports has not caused an obstruction within the chambers eg using an overlong screw thread? Regards Roger |
| Roger T |
| Check the carb needles are the same hight (shoulder level with the base of the damper). Remove the needles and check that they are embossed with the same profile number. Point of interest, what are they? One profile may have being tampered with in the area causing the richness. Swap the needles or borrow others to check results. Richard. |
| RH Davidson |
| Ok I'm back. It's the next WE now and I will try various things, starting with boosted fuel. FRM I'm sorry that I'm eliminating all the ideas put forward, but I'm looking for the esoteric, obscure ideas. I don't know the "piston drop test", if you could explain, please. Filters off. New FX needles, assumed to be correct. I can't accurately check the cam, as I don't have the equipment, but eyeball checking shows that the inlet opens 16 deg BTDC and closes around 60 deg ABDC, close enough to specs to be eliminated as a problem. When running with the rocker cover off, all valves, or more accurately, all rockers seemed to be doing the correct thing. If the fuel doesn't help I will check the valves, directly. Willyphixitt I have headers, haven't checked for blockages, I will leave this check till last, as I don't relish pulling the minfolds off again. Ray See comment to FRM, above. FRM It may not be detonation, but poor wavefront propagation of the lower octane fuel. Chris Will attach photo to this post and another to next post, annotated. Roger Thanks for seconding the fuel issue. Have you been using Shell V Power? Castrol, Oil? I think you mean Caltex, Vortex? Paul Maybe the differences are due to slight differences in the clearance volime, as the head is cast afterall. This may just affect the wavefront enough to give the differences. That it affects 1&2 differently to 3&4 may be just coincidence. The timing is spot on, the engine runs quite smoothly at idle, it's only at 1500 RPM that the shakes occur. Anyway by tonight I will have tried the souped up fuel and it all may be resolved, I hope anyway. Sandy No booster and all vacuum hose are new. The old PCV Valve let air be sucked because the diaphragm was scrunched up inside. Fixing the diaphragm caused it to suck oil, because the metering needle was missing. I didn't know this at the time I fixed the diaphragm. Roger I haven't specifically checked the butterflies, however, when operating the accelerator, even a tiny bit the piston move up at the same time and amount. I would interpret this as the butterflies being OK. Obstructions in the inlet. When I resealed everything I was already trying to solve this problem. It maybe that the screws fitted are too long, but they would only protrude into the balance tube. Richard Brand new FX needles, to replace the #5 s that were in the carbys originally. The FX is the lean version of the #5, and the ones specified for a GB engine. Herb  |
| H J Adler |
| Hi, Another photo of the other side of the engine Herb  |
| H J Adler |
| Herb, Yes, Caltex Vortex 98. To hell with the cost! Does the 123 distributor have a vacuum advance? If so, where is the connection to cab or manifold? - I don't think see it. Regards Roger |
| Roger T |
| The 123 is available both with and without vacuum advance. Vacuum advance gives better cruising economy and slightly sharper part-throttle acceleration, it's only all-out competition vehicles for which it isn't really relevant. Any distributor vacuum capsule can be connected to either the carb or the inlet manifold. Carb is preferable if you have a port for that purpose, otherwise inlet manifold. The only difference between the two sources is at idle and just coming off it. At idle carb gives zero vacuum advance for eaier starting, manifold gives near maximum advance for lower emissions. As you come off idle carb vacuum rapidly increases to meet manifold vacuum, which is changing very slightly. In theory manifold vacuum should reduce as it comes off idle, but in practice with the MGB the engine is more efficient just off idle, so the vacuum goes up slightly, and then gradually reduces to virtually zero as the throttle is opened to maximum. At all practical throttle open positions the two sources give the same signal. You can switch between the two sources on any engine without changing anything, which is exactly what the factory did on UK cars when changing to manifold vacuum in September 76. I really can't see minor differences in combustion chamber volume causing that much difference between shorting 1 and 2 compared to shorting 2 and 3. Causing slight vibration maybe, but your shaking is down to those differences when shorting plugs (or at the very least that certainly shouldn't be happening), it's the cause of *that* that is the conundrum! Did you swap over 3 and 4 and see what happened? I's been so long I forgot this shaking was at 1500rpm. Did you short the plugs at idle? 1500rpm with the shakes? Or both. If only one of them, which, and what happens at the other? |
| Paul Hunt |
| Herb, What has the centre tapping on the manifold been plugged with? Does it go down and interfere with the balance tube? |
| Chris at Octarine Services |
| Herb If the thing idles ok then runs that rough at part throttle but smoother further up in the revs I would have a real good look at your throttle linkage adjustment and make sure that the throttle is pulling on both carbs exactly at the same time. Balance the carbs up at idle so that they are both sucking exactly the same and then adjust the throttle linkage to make it so both carbs come off the idle position together-exactly and I reckon your shake will disapear---------I hope- Willy |
| WilliamRevit |
| Hi, Roger Yes there is a vacuum advanced, and it works, tested by sucking on the tube and the engine increased its revs. Paul, you are right about slight differences in combustion chamber volumes, but I'm now clutching at straws. I'm not prepared to swap #3 & 4 as #4 would be on its induction stroke when it would be fired, and I don't want to cause more problems than I already have. If you look at the second picture I posted you can see the vacuum hose crossing the fuel and PCV hoses and disappearing behind the damper screw on the rear carby. It goes to the port on the carby, as Paul described. Chris A grub screw. Willyphixitt The throttles have been set with a flow meter, and when testing with the slightest throttle movement both pistons move slightly at the same time. Further work and results. As I mentioned I would do I replaced the fuel with 98 premium. There was an improvement, but this only made the shakes more peaky, ie a much smaller rev range. I went back to the timing and at idle, ~ 7 - 800 rpm, it wandered from 0 deg to 15 deg. Whether this is a new symptom or I disregarded it previously I don't know. Given this wandering I cogitated on it and what could cause it. Something in the distributor drive chain? Checked the rotor button, quite acceptable backlash. What next? Timing chain and tensioner. Off with the timing cover again and remove the tensioner. It didn't pop out so I thought AH HA! Anyway cleaning and lubricating it, reinstalling and all that didn't make any difference. Well that's 2 more things that don't cause the problem. ( I sound like Edison and his many tries at making a light bulb) Rear plugs still black, front ones now look as if nothing is happening in the cylinders, ie very clean. The jets in the carbys are now level with the bridge! I am thinking that there must be something in the carby setup, but what? Today I had a bit of luck in that one of my clients said that they know of a whizz with SUs and will get his details to me. Hopefully he can sort out / tune the carbys. Herb PS I have been writing up all my issues with this B for the local MG club magazine, 8 episodes so far, running to 3 - 4pages A5 each and not counting this problem. If anyone would like a read or knows of a site to post I would be happy to pass it on. It is in WORD format, with pictures. |
| H J Adler |
| Herb, In the second photo you posted you have labeled an 'old PCV valve bracket'. The head on the mounting bolt looks high and it looks to be mounted on the intake manifold - how deep into the intake manifold tube does the thread go? I think that is pretty much in line with the feed from the front carb! Is there any chance that you are choking the flow into 1 and 2? I know you have revisited the protruding threads issue and seem relatively unconcerned by the possibility of any interference inside the balance tube, but I notice that Chris (above) is also focused on this point. If we really are looking for something from 'left-field' (Paul's conundrum)I think it may be worth drawing the sealing threads/bracket securing bolt, if only to be able to cross those off as possible contributing factors. Its a long time since I had my own intake manifold in my hand but I find myself pondering the possibility of a protruding thread 'catching' fuel from the air/fuel mix and dripping it to the balance tube floor. Disruption of supply to the cylinders? Backfiring within the balance tube? I don't know but the thought stays there! Regards Roger |
| Roger T |
| Another wacky thought - could there be leakage across the plug wires if they are not separated? - we are grasping at straws here. |
| Art Pearse |
| Art's thought is relevant to V8s where 7 and 8 are side-by-side and fire sequentially, in that case the correct routing is to swap 6 and 7 over through the combs, but as 4-cylinder wires are relatively short, widely spaced and don't go through combs it isn't really relevant, unless the cap is faulty and breaking down internally. Swapping 3 and 4 for a short period really isn't going to do much harm, these engines aren't that delicate. |
| P Hunt |
| Hi, I've not done anything with the engine for a while, but to answer some of your comments. Roger That bolt is actually a sump plug, 5/8" UNF. It is about 3/4 the length of the original fitting, ie about 18 mm. The balance tube is about 25mm in diameter and across the top of the inlets, with raised bosses for all the screws etc., so even if the bolt did protrude into the manifold it wouldn't be in the path of the inlet flow. Art Thanks for the whacky thought, but unfortunately the leads are well separated, see my photo of the ignition side. Herb |
| H J Adler |
| Herb I have been following this thread and see that most areas have been well covered. One thing I notice however is you dont make mention of when this problem started or something that could help with diagnosis "how it drives on the road" power,missing etc. I would not leave that breather system like that without at least having a PCV valve in line. You may have covered these points and I might have missed it but if you have not lets know. Denis |
| Denis4 |
| Hi Denis, I bought the car last January as 95% complete to concourse standard. I have spent the last 11 months sorting out all the mechanical problems and didn't pay too much attention to the way the engine ran, as this would be fixed once the tuning is done. So I have never driven the car and as far as I can tell it wouldn't have been driven for some time previously. I haven't fitted a PCV valve, but it has been in the back of my mind to do. Thanks for your thoughts Herb |
| H J Adler |
| Update Had an MG club mate over, who has many years of experience of tuning SU's and he suggested that cleaning the plugs with a mini sandblaster scores and degrades the porcelain insulation and I should get new plugs and only clean them with a wire brush. Will try this. Herb |
| H J Adler |
| I've never cleaned a plug in 40 years, if everything is as it should be they shouldn't need it being self-cleaning. New plugs should be an early diagnostic step in what appears to be mis-firing *causing* fouling. |
| P Hunt |
| Herb, I'm confused. In your first post you said it had new plugs. Maybe I missed something later? Charley |
| C R Huff |
| Hi, They are new plugs, but after a bit of running they are black. I have been cleaning them with a special plug cleaner, which is a mini sand blaster. Apparently what can happen is that using one of these cleaners abrades the insulation and can cause misfiring. My mate suggested cleaning them with a wire brush instead, as this does not affect the insulation inside the plug. Herb |
| H J Adler |
| Herb - Your mate has it backwards - sandblast will eventually abrade the porcelain (very slowly, since the insulator is primarily aluminum oxide), but will not affect the insulating qualities; wire brushes leave minute deposits of metal on the insulator, which can cause misfire. You can see the metal as a grey discolouration after brushing. In years past I cleaned the unobtanium 10mm plugs on my AC Bristol with nitric acid, since it dissolves lead deposits, but that's not an issue today. I now use a propane torch to burn the deposits off - don't forget and pick the plug up just after you shut the torch off!!! FRM |
| FR Millmore |
| Herb, I'm with FRM on this one. I have been told never to clean plugs with something metal for the reason cited. That is, the residual metal on the insulator makes a path for the spark to short out. Sand blasting is accepted practice on aircraft spark plugs (which are quite expensive) and the only caution I would add is to inspect them very carefully for any sand that may wedge itself in the plug. Charley |
| C R Huff |
| Herb where in Victoria are you Denis |
| Denis4 |
| Hi, Charley I always blow air at 100 psi through the plugs after cleaning. Interesting about cleaning with metal and that sand blasting is OK! Denis I'm in Geelong. Herb |
| H J Adler |
| Herb, Ok, with you on the manifold plugs, though I would think that a plug 18mm in length into the balance tube is somewhat deeper than that on mine. I will draw mine this weekend to let you know what I have. Now. You have rebuilt the entire intake/fuel side of the engine. You have investigated and analysed the entire electrical side of the engine. Not dismissing a possible origin of this problem being within the engine and a consequence of a combination of the two sides, BUT, the dimension of the shake you describe would suggest a very significant 'internal' origin. Your assessment of the timing of the valves suggests that the cam is not the likely problem. You have checked the timing chain and if it were something like bearings you would expect there to be significant running/exhaust evidence. You have also stated that on a change to 98 RON petrol " there was an improvement, but this only made the shakes more peaky, ie a much smaller rev range." What is the nature of that change? I am starting to wonder about mechanical balance issues. Your first thread stated "supposedly restored to 95% concourse standard." This may be a case of 'telling my grandmother how to suck eggs', but are you sure that the gearbox mountings are both complete and secure ie that all bolts (cross-member to chassis, rubber pads to both cross-member and the box housing) are all there and that all bolts are truly holding in the threads into which they mount? Are you equally sure that the bell-housing to engine bolts are all there and secure? May seem trivial but a member of our club had an experience where work done by a 'reputable' garage left him with just three bolts housing to engine!! I know all of us are feeling your frustration with this problem. Hang in there and we can see it solved. Regards Roger |
| Roger T |
| Herb - It is possible that you should just bite the bullet and strip the engine. You've got so much in it now, with apparently more than ample evidence of incompetence afore. A friend once had a Volvo 122S that we could not eliminate a shake and running roughness on. The engine had recently been "overhauled" by a Volvo dealer. It was rough from the day he picked it up, but of course not as rough as before they did it. We finally pulled the head and found the two center pistons to be +.060, and the end ones Std. Of course, on a 4 cyl engine, this means the heavy pistons were both going the opposite direction to the two light ones, and two holes had higher displacement and compression than the other two. I'm even more concerned about the possibility of something bolted together wrongly - or loosely! I've taken a number of "just rebuilt" engines apart before running them, and saved a lot of good and new parts by so doing. More than one would have suffered total destruction if I had started them, and in some cases there probably would have been no warning. I got an Austin America like that - all new engine and gearbox, found a gearbox thrust washer under the back seat, but no other extra bits. Took it apart to find main caps snug not tight, rod caps mixed and not correctly tightened, new oil pump loose, cam timing out, and several critical gearbox bolts loose, and of course a gap where the stray TW went. Put it back together, wore out a couple of cars with it, and it sits on a shelf now waiting for a new body. FRM |
| FR Millmore |
| Plugs going black after a running a while can be caused by not firing properly, or too cold, or too rich a mixture. But after that fouled plugs won't fire properly anyway. This *still* sounds like not firing properly. |
| P Hunt |
| Herb, FWIW, that plug on my car has measures as follows; plug thread depth of 8mm plug thread below the brass washer 7mm thread depth in manifold 9mm (approx) Additionally, the plug is hollowed out to a depth matching the underside of the plug head. I don't know the physics of fluid/air mixtures, nor the extent to which the mixture moves front/back within the balance tube. I would suspect though that there is a reason for both why the threads are contained within the raised housing and the plug is hollowed - it seems almost as though the designers wanted a 'head-space'. I have not pulled the remaining plugs on mine as I suspect what I found will be repeated on those. On that basis if your plug is around 18mm it is protruding about 10mm (1cm) into the mixture space. I still wonder about the risk/degree of those hanging threads 'scavenging' fuel from the mixture, subsequently dropping it across the opening toward the floor. I noticed on my plug a small amount of soot, leaving me wondering whether there is a minute degree of 'open' exchange between the inlet manifold and exhaust valve. People do talk about backfiring through the carbs! Do you think it worth cutting that plug back to say 9mm to see if there is any change? Are any other plugs hanging into the balance tube? Regards Roger |
| Roger T |
| Hi All, After several weeks of pre Chrissy functions I have had a chance to get stuck into the engine again. This time I think I have found the problem, and as usual the answer was quite simple. What I did is to replace the 123 dizzy with the original one and the shakes disappeared, though engine still ran rough. Note that the previous time I swapped the dizzys I still had other undiscovered issues, like the faulty PCV valve. OK, either the 123 dizzy is faulty or I've chosen a very bad curve. As they say, "Back to the drawing board". Out with the 123's specs and the Lucas dizzy curves. Yes they are very close. The installation instructions for the 123 say, roughly, "align the TDC mark with the alignment mark". So I set up the notch in the pulley with the TDC mark on the timing cover. Whilst perusing the curves for the dizzys (the first time both together and in detail) I found that at higher revs the 123 didn't have anywhere near the advance that the old dizzy had, about 10 deg. Big head scratch. Look at the curves again, the Lucas ones were for the dizzy then had to add the static advance, but the 123 had this static advance built in, ie 10 deg, from 500 to 1000 rpm. I thought about setting the 123 10 deg advanced so that the higher rev advance was correct, but was very wary as the installation instructions clearly stated that the alignment mark had to be set to TDC. Now the penny dropped. What if the TDC mark is the notch on the pulley and the alignment mark is the static 10deg mark on the timing cover. This would bring the 123 advance to the same as the Lucas. Out to the garage and swap dizzys again, set the 123 to 10 deg advance and start. Engine runs much smoother, though still a bit rough, probably poor tuning, but no shakes. When I previously checked the timing with a strobe I checked against the 123 specs and they were OK, but the specs are just for the dizzy, not for the crankshaft. A case of different understanding of terminology led to many months of frustration. Started to tune according to the good book and ran out of petrol. Roger, Thanks for your thoughts on plugs protruding into the balance tube and description of yours. Mine is about 18mm long in the thread, of which about 6 mm is taken up by two washers and the old PCV bracket, one washer above and one below this bracket. The plug only has about 4 turns of thread holding it in. Thanks to All who have contributed to this discussion. Herb |
| H J Adler |
| Hi again, Maybe the above post is all hogwash. I've just received an email that the 123 must be set to TDC on the timing cover. Now totally confused. Herb |
| Herb Adler |
| Herb, Just a thought to add to all you've already been through. If your distributor advance isn't right, and the chain of dimensions is close to the limit, it could be that as the advance changes at the partial throttle opening for 1500 RPM, it could get to the point where the rotor arm is in between the contacts inside the cover, and could be sending the spark to the wrong plugs. Might be that they are just OK for 1 and 2, but not OK for and 4. So check with your timing light that you get exactly the same behaviour for 1 and 4 as well as 2 and 3. And a Happy New Year to you and all reading this. David |
| D Balkwill |
| Under centrifugal advance the relationship between rotor and gap doesn't change, only under vacuum advance. After-market electronic triggers for MGB 25D and 45D often seem to change the relationship (spark phasing) as several people have noted they have had to move the distributor quite a bit after fitting them to restore the timing. In a couple of cases I'm aware it was so far off the spark was occuring between cap contacts as mentioned and had to be scrapped. For that to be happening with a 123 then the distributor itself is faulty - quite possible of course. Indeed the 123 must be statically set to TDC, the electronics then apply (or should apply) the correct static advance as well as the correct curves. Have you checked your timing mark? Pulleys can delaminate and shift timing. Fix a probe into No.1 plug hole such that it contacts the top of the piston shortly before the piston reaches TDC. Turn the engine so the piston just touches the probe, and make a mark on the pulley corresponding to the TDC pointer on the cover. Turn the engine backwards almost 360 degrees until the piston just touches the probe again, and make another mark on the pulley. True TDC is mid-way between your two marks. But I don't think this is an issue of timing, you can go from grossly retarded to over-advanced and the engine will still run pretty smoothly albeit with occasional slight misfires when over-advanced. All along this has seemed to be the rear two not firing, either because of no spark or no fuel. |
| P Hunt |
| The two major anomalies are the wandering timing and the fact that the rear plugs can foul in such a short space of time - and under no load. Assuming a 4 degree timing variation just about acceptable at idle, a 15 degree change might point to loose timing gears or a bent or binding shaft, loose bearings or excessive endfloat. Look at this video showing points scatter. Your target should be within a couple of degrees http://www.youtube.com/watch?v=F1PFb46gKlc Certainly this wants correcting before moving on, and substituting the 123 for a known distributor might be beneficial if only to eliminate a possible culprit. The sooty plugs suggest more than overfuelling, although a mix of 90 with 100 thou jets on the rear SU would almost fit the bill. You might look at oil fouling 3 and 4 due to the lack or damaged inlet valve oil seals or even broken springs. On a recent post you mentioned the shakes had disappeared - but motor still ran rough. As you increase the revs past idle do the dashpots raise in unison all the way through the rev range? Or do they chatter up and down or does one drop and pick up? Till the carbs are working together I'd leave off the PCV. On an engine with new bores and pistons crankcase pressurisation will only add another variable. |
| F Pollock |
| Hi All, I am currently lazing on the beach, hence have not done much with the B. Following on David B's suggestion I was thinking about how to measure, and maybe correct, a problem with the rotor being halfway between two contacts. (I had checked the ahrmonic balancer, as per Paul H's suggestion and as close as I could measure it was OK.) As a possible remedy I was thining of creating a kick back on the brass contact of the rotor, something like on the original Lucas. As I had to take my daughter home (only 30 mins up the road) for work I decided to spend a bit of time scratching my withdrawal symptoms and buy a second rotor to modify, so that I still had an original in case I stuffed up. The first thing I did was to confirm that the new rotor actually fitted, it did. Then I went in to check the angle the brass wiper swept, as a matter of curiostiy and to see how much of a kick back I would need. The measured angle was 30 deg, which should have been more than adequate. Whilst messing with the rotor I noticed a small black spot on the brass near where the centre contact would sit. A bit of dirt, I thought, rub it off. It didn't rub off. Getting a magnifying glass I had a good look and there was a tiny black spot on the insulating part, next to the spot on the brass. Looked underneath and it appears as if there is a corresponding black spot. Bells started to ring - a punch through in the insulation, which would cause all sorts of problems with the sparking. I haven't tried running the engine yet (going home tomorrow) but have a gut feeling that this is the root cause of the shakes. I will keep you posted as to results. Herb Isn't modern technology marvellous, here I am at the beach, everyone has gone off somewhere, giving me a bit of time to catch up. With a laptop, mobile phone internet enabled I can "talk" to the world. |
| Herb Adler |
| and the world is listening, waiting patiently for the solution to the problem. Richard. |
| RH Davidson |
| Normally a rotor would affect all cylinders if itwas tracking through, i.e. the engine wouldn't run at all. A cap, leads or plugs is normally the cause of some cylinders not getting a spark. But in either case, except for the plugs or the leads shorting by the plugs, it would be revealed by a timing light flashing when on the coil lead, but not on one or more plug leads. Remember it is vacuum advance or incorrect positioning of the trigger or points on the points plate that changes the relationship between rotor and cap, not centrifugal advance, or general timing. |
| P Hunt |
| I have been following this saga with interest, unfourtunatly I have nothing to add, but I can't wait for the next instalment, hope it is good news. |
| D G Levy |
| Hi All, Well I'm back home and have been playing with the B. I replaced the rotor and also set the timing curve to one that I think is closer to the B specs. (Now that I understand timing curves). Timing spot on at 1400 rpm (15 deg). The engine runs smoother, with occasional tremble with a misfire, no shaking at 1500, until I push the rear (cyl 3&4) carby's piston up a bit, then shakes start, and stop when released. Front carby _ no shakes. Cyl 1&2 plugs clean ( see pic) 3&4 black after only a few minutes running. Tried what Paul H suggested some time ago, ie swapping float bowl covers. Required a bit of re-hosing, as they are mirror images. No difference in performance. Tried swapping the Pistons and their chambers, no difference. The jet on 3&4 is level with the bridge, 1&2 slightly down. Double checked the jets again, and guess what? The front jet won't accept a 2.5mm drill - OK, but the back one will. Now 2.5mm is a smidgeon less than 0.1". So F Pollock, above was right on the ball with his suggestion. So get the old jets and use one of them. Ha Ha, guess what? One is 0.09" the other is 0.1". I am really p...ed off having bought two reco kits for the carbys, both the same part number, but one with 0.09" jet and the other with the 0.1" jet. No wonder I was getting nowhere with both sets of jets. I guess I was unlucky in that whenever I measured the new jets I happened to pick the OK one. The other had to be the same, didn't it? After all, new with the same part number. Oh well, now to centre the old jet and re-try.  |
| Herb Adler |
| Talk about bad luck! RAY |
| rjm RAY |
| Hi All, Just come back from swapping that 0.1" jet for the correct one. I took my trusty 2.2mm drill bit with and checked the supplier's stock of jets, all were OK. Just my luck to get the one that was off. Previously I said I would use one of the old ones, but the one with the 0.09 hole was of a fabricated construction, ie an aluminium tube with a brass plug in the end, drilled to 0.09". Unfortunately it was so worn (3 thou under, compared to the new one, and when you consider that the min clearance of the needle to the jet is something like 1/2 thou) that I couldn't centre it so the the piston didn't jam. It was OK until the choke linkage was connected and then the side pressure offset it. Now to go and install. Paul H. Your comment about the rotor to cap alignment not changing with centrifugal advance is correct for a mechanical dizzy, but for an all electronic one, like the 123, it isn't. These dizzys don't have any mechanical advance, it is calculated from the engine rpm and the spark is fired earlier in the rotational period, ie with the rotor more advanced. Herb |
| Herb Adler |
| Fair enough, I hadn't thought about it before, but it makes sense for the 123 to have a solid shaft and not the split shaft of mechanical distributors. |
| P Hunt |
| Hi All SUCCESS SUCCESS SUCCESS!!!!!! It works. I could almost balance that proverbial penny on it now, at 1500rpm. Thank you all for your suggestions and moral support. It helps to have a "mate" to talk to and occasionally a shoulder to cry on. Herb |
| Herb Adler |
| WOW!! Congratulations Herb. This is the classical 10/10 repair. 10 hours of teeth gnashing diagnostics and 10 minutes to fix. For the record, please share with us the ultimate repair scheme that finally resolved your issue cheers Gary 79 MGB |
| gary hansen |
| "This *still* sounds like not firing properly" Paul Hunt, post of 12 December. My hat goes off to the man, once again! And congratulations Herb on sticking with it to the successful conclusion. Regards Roger |
| Roger T |
| Found it at last, well done – pat on the back and all that. Remember - similar items are seldom identical, you just haven’t found the difference. Richard. |
| RH Davidson |
| Hi Gary, Partly it was pure luck. I was writing a reply tp this thread about the jets being OK. To back up my claim I went and measured one, luckily the crook one, using drill bits. A 3/32 fitted, but a 7/64 didn't. I then did the calcul;ation to convert it to decimal and 3/32 came to .0935xxx, ie bigger than .09. Double checked using metric bits, 2.0mm fitted, 2.5 didn't fit the front jet, but did fit the rear one. 2.54mm is 0.1" Roger Paul and others did suggest fuel problems and I double checked everything, float levels, blocked, unblocked fuel lines, even blew out the jets, replaced the float needle valves and so on. However, I checked a jet and it was OK. I erroneously assumed that the other was OK too, new, out of two reco kits with the same part numbers - of course they were the same - ha ha. Today I've fitted the air filters, with a mod I made so they can be replaced without the whole assembly falling apart, see pic. Also you can just see the PCV valve I've installed in the top right hand corner. Richard I know, but when do you stop checking everything and take some things on faith? Taking checking to the limit and you'd be checking the diameter and pitch of every nut and bolt. Herb  |
| Herb Adler |
| I didn't have the job of doing the hands-on for this problem of course, but I can't help thinking this should have shown up straight away as a failure to get the correct mixture on the rear carb, even with the jet wound all the way up. |
| P Hunt |
This thread was discussed between 02/11/2009 and 27/01/2010
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