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MG MGB Technical - A question about tuning
|As some of you might know, I recently tuned up my engine using a gunsons gastester. Although the engine is running great, I was wondering today when I was driving did I do it right. I forgot about that bloody bimetallic strip. So the manual says set it to 3% CO, which I did. But is that before the bimetallic strip kicks in or afterwards, cause I did it afterwards.|
At normal operating temperature.
|Great so I did it right then. Thanks. That puts my mind at ease.|
|My car is still using a lot of fuel. It must be something to do with timing or mixture. What I find interesting is when Im starting the engine from cold, I put on the choke to get it started and then as soon as the engine starts I turn off the choke completely or the engine wont run. If I try and rev the engine quickly, it will die, so Ive to gradually increase the revs, then I can rev the engine a few times quicker then it will run fine. Is this normal? When the engine is idling as well and I put the timing light on it, the mark hops around quite a bit. Its a bit lumpy on idle as well. It does drive well though. My distributor is a 123 electronic distributor. I'm wondering is it faulty. Maybe I have it on the wrong advance curve either. I have it set for my engine though according to the manual. The engine type is 18V847. Its got 87000 miles on it and is original to the car. Compression is 140, 130, 130, 125 on it from 1 to 4. I also seem to find myself adjusting the tappets quite a lot. Ive checked and rechecked the timing and done it properly this time and yet I still find myself being completely unable to get better than 25mpg, and thats if I drive like a granny. If I drive it normally Im getting around 18 to 20mpg (and I dont rally it either. I drive it like any other car). This is on normal roads, not stuck in traffic and moving at a reasonable speed. I bought a bottle of fuel system cleaner to try in it to remove any buildup on the cylinder head. The engine doesn't overheat, I get good oil pressure, the carbs aren't overflowing into the throat and it seems to pull well without hesitation. I know Im harping on about this issue but I am still sticking with my guns that this car can get much better fuel mileage than its getting. I dont like having to drive like a granny. I just want to enjoy the car but when I find myself watching the fuel gauge and the price of fuel spiralling out of control, Ive been using my brothers mazda 323 at times. My entire fuel system is either new or reconditioned so there's no leaks. So Ive provided all information I have on my engine in order to try and get an answer to this conundrum. Please help!|
With that variation in compression and the fact that you have to adjust valves very often I think that you have a bad camshaft and that #4 lobes are wearing faster than the others. Try pulling a compression test and immediately remove the plugs and introduce a tablespoon of oil into each cylinder. Then retest the compression. If they all come up in pressure approximately equal then you know that its not rings.
Which set of valves require setting more often? This will tell you the offending lobes.
If so then you must replace the camshaft and all the lifters at the same time. Do not forget to use an engine oil supplement during break-in of the cam. It is most urgent.
|They all seem to need adjustment at the same time. The last time I had to adjust them there was a horrendous amount of lash. I could grab the rocker and move it around 2mm or more. I was afraid you might say my cam is on the way out. This I suppose would also explain the lumpy idle and the jittery timing mark cause the timing gear on the cam could be worn as well. Well the bottom line is I dont have the money to pull out my engine and start replacing parts in it. I'll have to live with a worn cam for the moment. My main concern at the moment is the fuel consumption though. Can I do anything about this?|
|With a worn cam there is nothing much to do to increase fuel mileage other than riding a bike or walking.|
|*Sigh* Right, Im getting another car then. I'll keep the MG but Im going to get another car as a daily runner. When I have a bit of free time and money Ill get the cam replaced, but I intended to do some engine work on it anyway. I was looking at an econotune head from Peter Burgess as well. I wonder would it be just better to replace the engine.|
Its possible that your thoughts on the cam are accurate - and a contributor to the fuel economy issue. Its also possible that you are a little over-fixated on the economy issue and overlooking the age and condition of your car relative to what you might reasonably expect from the results.
However, its also possible that a part of the economy equation may be the condition of the rocker system ("there was a horrendous amount of lash"). This can be rebuilt by the amateur, I know, I have done it.
Given it's the system that controls valve opening/closing, and short of pulling tha whole thing apart, is the component whose refurbishment can make a real contribution to overall engine performance short of a bottom rebuild.
I suggest you check it out, new shaft, bushes (they can be removed and replaced with a simple bolt/washer/shifter arrangement), drilled bushes (yes, if I can, you can), soldered/brazed closures and before you know it, the inlet/exhaust controlling system is a whole new equation.
Its a little late here and we have just had our daughter around for a little career ego stroking/reinforcement, so if I sound a little jumbled or miss spelling cues - its all down to a few glasses of good old Hunter Valley red!!!
Wish you well with this as I have been following each of your posts and I have to say I have a real respect for your determination and effort with this car. I'm sure it is worth it all and really respect the fact that you stick at it as you do. So, to borrow from the movie business, you have had a personal impact on the 'far side of the world'. Well done!
|Well believe it or not I do actually enjoy working on my car. I always have done. Diagnosing and repairing problems gives me a great buzz but its just that its costing me money I dont have. I do really like the car though. I was thinking just now of a way of measuring the wear on the camshaft without removing it. If I remove the rocker shaft and turn the engine so the pushrod is at the bottom of its travel and then measure how far up it moves as I rotate the engine. Then I can compare that to how much its supposed to move and by knowing that I'll know if the wear is in my rocker shaft or cam. I expect a bit of wear there. I mean there'd have to be after 87000 miles. I just want to find out if its excessive. So can anyone tell me what the lift of a new camshaft is then? If I know my cam is alright then it will really put my mind at ease cause it will be a fair indication that all is well in the bowels of the engine. Any top end work I can easily do myself. Ive no facility for removing the engine though. Keep throwing advice my way! I'm learning a lot as I go along and my car and I thank you all for the advice! Its coming together.|
|Well, when I had the engine of my '69 out four years ago I removed the sump to check end float and to do a visual check of the interior. I knew the pressure test had returned good enough figures (160-170 from cylinders 1 -4) but was interested in the condition of the interior of the block. All seemed very good from that investigation. This despite the fact that I know I have a degree of ovality in cylinder no 2).|
I resolved that the condition of the bottom end was good enough for the use I intended for the foreseeable future - that of daily driver and occasional weekend tripper. I know it needs a rebuild, but from the way it drives at present I am confident I can put that off for a while.
What I did do was replace the head (because the existing had a hairline crack from beak to valve seat area in number 2) and rebuild the rocker system. The difference in performance was a marked improvement. Recently I checked her disposition to run - comfortable at about 90-95 mph on a local B road. Certainly she was sounding like she was working but no trouble in doing the speed. Temp and oil pressure behaved as per normal. Mileage at present is 129,000.
What is the condition of the cam? Won't know until I have opened her up. But while she will run as she does I am not overly concerned. The rocker rebuild made a significant difference to the car.
Post the head rebuild she returned 39something mpg on a 200 mile day run around the local district. I have since had the carbs rebuilt and am currently playing with timing to get things right. Only then will I have a relaible indication of the economy figure. I suspect it will be somewhat less that that above but think that setting may have been a contributor to the cracked head ie that the POs servicing may have simply attempted to cover declining settings et al rather than address causes.
|Well if I was getting 39mpg at all I'd be very happy. I was half thinking of removing the head and taking a look inside. In the process I'd replace the head gasket of course. In fairness, one piece of info I neglected to mention, its a leaded head because its stock and Ive been disregarding this fact and using unleaded fuel without an additive. My valve seats could be all recessed and not sealing properly as a result of this. Would this be the type of thing that would have a short term or a long term effect on the engine?|
Yes, you can check cam lift with a dial indicator and magnetic base. You could do it at the valve, or you could remove the rocker assembly and do it as you mentioned. Sorry, I don't know the numbers, but all intakes should be the same, as should all exhaust.
If you add a degree wheel to the dial indicator, or make marks on the front pulley, you could check the cam timing.
Do you know if you have the original (or original replacement) cam? Maybe someone put a custom cam in it and didn't time it correctly. I don't know about the UK spec cars, but I believe that the US spec cam for an 18V engine was not thought of highly. Also, I believe the 18V engine has a single cam chain system rather than the duplex of the older engines. This may cause faster wear, which would change the cam timing. It can be converted to duplex.
Wear on the rocker shaft and rocker bushings is quite common. If it is really bad, you can feel it without disassembly. I think there is an oil plug in one end of the rocker shaft. If I am right, and if it has gone missing in action, this could reduce lubrication and increase wear.
If you want to go to the trouble to pull the oil pan, which can be done without pulling the engine, you can visually inspect the cam.
If you decide you need a new cam and cam followers, I am reasonably sure that this can be done without removing the engine, though I have not done it. You would have to pull the pan, rocker assy, radiator, timing cover, distributor and drive, and oil pump. You might also have to remove the valve chest side covers (behind the manifolds).
If your engine is making metal, an oil analysis of your used oil should show how much and what kind.
|C R Huff|
I don't think you need to be overly concerned about unleaded fuel and valve recession in the short term. Many here have contributed experience of running for considerable mileages without any adverse affects. Leaded fuel had impregnated the head material with a degree of lead that lasts well into the period of unleaded fuel use - 'lead memory' I think it is called.
Depending on how previous owners have driven (and serviced) the car, there might be some wear in the head. I would expect that if that were significant that it would show in a degree of leaking down in a compression test ie valve seats worn.
Of course, the only way to tell is to remove the head. That was the choice I faced. I chose to remove it only to find the valve seats had hardened inserts in place - but I did pick up the crack in the head so it was probably worth doing.
If you do remove the head, and it might be a sensible thing to do especially if compression readings suggest the bottom end is serciceable for the time being, I would suggest using a Payene gasket on reassembly. I did and, without retorquing, have had a perfect seal. I think they are superior to the older style of gasket, at least for normal road use.
Sorry, but I think I do need to head off to bed - its getting quite late here. I'll check in tomorrow to see where you are at. Think things through carefully and keep a sense of perspective about things like fuel prices, at least in the short term. You can time work and expenditure as you wish.
|OK so I have a bit of work this weekend then. I'm going to get the rocker cover off and remove the rocker shaft and inspect it for wear. I'll also measure the lift of the cam and see how that is. I strongly doubt there's anything other than a stock cam in there. The engine is stock in its entirety everywhere else and it would seem silly to replace the cam and not do any head work. The UK spec cams are supposed to be good actually. The US spec cars I understand do have a crap camshaft. So we'll see how my inspection goes and then Ill have a good idea as to what condition my engine is in in general. If the cam is bunched, I think it would be fair to say the engine is well worn and will need a rebuild soon. I'll also be interested to see if this fuel system cleaning stuff I got does anything. If my valves are all coked up, this might be just the job to get a bit more from the engine.|
I would not assume that a worn cam means the engine needs rebuilding, however your compression figuers don't look ideal. A cylinder leak down test should tell you if the compression loss is valves or rings.
Then again, a bad cam or an out of time cam can cause low compression by not letting enough air into the cylinders, or letting it out at the wrong time.
When I first got my GT it had 2000 miles on a rebuild, and it had 105 psi in every cyliinder. I took a guess at cam timing because I had no specs for the custom cam, I moved it 18 degrees, and the compression went up to 145 psi. In the end, I had to rebuild the engine, but that was because the previous rebuilder did almost everything wrong.
|C R Huff|
|Well its not burning oil so on that basis it would seem more likely to be the valves. OK it uses a pint of oil every 1000 miles or so but I consider that very acceptable. Ive had newer cars that use more. What about the head gasket? Could that be on the way out and not be showing the characteristic signs of oil in the coolant etc? Or could my head be cracked? I dont know, Im thinking pulling the head off might not be such a bad idea. Although I reckon that would be a job I'd only want to do once, and if I did have the head off I'd like to be replacing it with a new econotune head. OK, let me take a look at the situation with the cam this weekend by measuring the lift. I'll carefully inspect the rocker shaft for wear as well while its off and then I'll go from there.|
|Ok I removed the rocker shaft today. Theres only a tiny amount of wear on it. Hardly noticable and theres no play in the rockers when theyre on the shaft. Using a set square I took rough measurements of my cam lobe lift. Considering lobe lift should be .264", heres what I found...|
Average over all 8... .266
1 - .256
2 - .295
3 - .236
4 - .275
5 - .275
6 - .275
7 - .256
8 - .256
Bear in mind these measurements are pretty coarse. There could be a sizable enough margin of error in there, but what this has all told me is that my engine is generally in good condition. I have the rocker shaft all cleaned up and ready to be refitted. So then the next thing really I suppose is to remove the head and see how the valves are. Has anyone got any other theories as to why the engine is using so much fuel? Would spark plugs make much of a difference. Currently Im using 3 tip plugs, to compliment the high voltage coil I have in it.
|I think there might be a small leak in the exhaust. Would this make a huge difference to fuel consumption? I only noticed it today when I was checking the CO level again. Im not even sure. Its hard to tell. My valves weren't much out of adjustment today actually. I quickly checked before I removed the rocker shaft and they were reasonably OK.|
On the 24th you said you could grab the rocker and move it 2mm or more. An inspection of the rocker shaft and rockers reveals no wear of consequence nor any play in the rockers. Where do you think that earlier movement was coming from? Could there be a problem with the rocker mounting studs eg are threads worn, nuts loose or working loose?
Are the lock nuts on the rockers holding or are those nuts working loose (from wear eg)?
Certainly I would expect any leak on the inlet or exhaust side at the engine to have an effect of some sort on tuning/economy. I know you have rebuilt the carbs and fuel system but did you check security of all fittings and ensure gaskets were sealing throughout both manifolds as well as carb components?
|No I should have been more specific. The movement was valve lash and once I adjusted the tappets the movement was gone, but it had only been a couple of thousand miles since I had adjusted the valves before that and they seemed very loose again.|
|Ross, I think this is all pointing to a conclusion you do not like. Valve seat recession, it's a real problem and does happen once the lead memory has worn off. In my experince a small leak in an exhaust does not affect economy much. Let us know what you find when you remove the head, however the work around may be to reseat tyhe valves and use one of the lead replacement additives.|
|I found I was having to alter my valve clearances frequently as well, although in my case it was only the back few valves. Eventually I discovered that at the strict 'rule of nine' point i.e. when valve 1, say, was fully down which should be the right time to adjust valve 8, the clearance on valve 8 was still changing which meant that unless I checked it at *exactly* the same position of the cam every time (which simply looking for its partner being fully down doesn't really allow you to do) I got a different clearance every time I checked. In my case sometimes they were bigger than they should be and sometime smaller. By turning the engine very small amounts one way and the other I found the point at which the gap was at it's maximum, i.e. it stops moving between getting bigger then getting smaller again, was one side or the other of the strict 'rule of nine' point. I now adjust at these points of maximum clearance - or rather I did do but haven't had to change them since.|
I'd be surprised if it were valve seat recession, as that makes the gap *smaller*, not bigger, if you think about it - valve sinks into the head, or rather rather rises up relative to the engine when installed in the car. Increasing clearances, if that is what you really are getting and not as above, would be caused by wear on the cam, followers, push-rods, rockers or valve stems, not the valve seats or valve heads. Cam wear can be detected by measuring the amount of lift of the push-rod or rocker each time you adjust them and find them increased again. If that is stable it must be one of the others.
|Yes Paul you are right that recession will drive the valve into the head and reduce clearence, thankfully I have never had to consider this before. Once the valve cannot close due to no clearence it will very quickly stop being a valve in that it cannot affect gas flow. I always watch both the rule of 9 and the target rocker closely and it seems to work well on my Oselli 270 cam. (Plugs out and a socket on the crankshaft pulley) Of course if the cam followers ( or something else eg rocker arm contact points ) are wearing rapidly but not the cam lobe you would get this situation, lift would be the same but clearence would increase. This is a puzzle, an MGB in good condition should return over 30 miles per imperial gallon. They are prone to dragging front brakes due to pistons rusting (I heard of one MGB GT that failed to complete a road run this year when a caliper burts into flames) and that can't help economy.|
|After I fitted the rocker shaft and adjusted the valves again, I took it for a proper fast drive giving the engine lots of revs to get it well up to temperature. While I was doing this I had my little gastester gadget attached to a spare battery calibrating itself and I got back and hooked it up to the car. The mixture was 1.2% more than what it should be, so I leaned it out a little bit to get it down to 3%. Ive been driving the car all day today and to be honest there seems to be a noticable improvement there, but I'll reserve judgement for another while. Its still running very well anyway. Im eager to see now if my tuning work has had something close to the desired effect. Im still hoping to get myself an econotune head soon though anyway, when I have some spare cash.|
|I always found a dial gauge and magnetic base were the easiest way to see when the valves were fully up or down. They can be had pretty cheaply these days and for a job like this you don't need one of the highly accurate Japanese ones. Just a cheapie seemed much better than trying to eyeball it!|
I think I originally got mine for doing the front wheel bearing end float.
I had a cam grinder person tell me to adjust the valves when the lobe tip was at 180 degrees from the cam follower.
The discussion arose because I was getting more clearance just before I approached the lifing part of the cam lobe. He said to ignore that (i.e. don't adjust it at that loosest point) as that extra clearance had to do with the ramping profile of the lobe.
That explanation kind of flys in the face of the common technique used on old Chevys where you adjust them with the engine running. But I took his word for it since he ground the cam, and it wasn't for an old Chevy.
|C R Huff|
Do you have a make/model number for the dial gauge you mentioned, or a supplyer in this part of the world?
I've thought of the advantages of these each time I've done my rocker adjustments but imagined they would be specialised and expensive.
|Roger, I got the gauge off eBay a few years ago now. Search for 'dial gauge' (or 'gage' since the yanks can't spell :) ) and you should see them. The magnetic base I got locally on sale at a local tool place that specialises in things like drill bits, tap and die sets, tool steel and so on.|
I don't think you need anything super accurate for the kinds of things we'd be measuring on a MGB!
|Ross - where in Ireland are you? If in Northern Ireland I'd expect the emissions standards to be the same as for the rest of the UK i.e. 4.5% if first registered on or after 1st August 1975, visual check only after that. I can't find if there is even an equivalent to the MOT in Eire, let alone what any limits there may be.|
A friends MGB just failed the MOT on brakes as they were pulling quite noticeably (to me anyway) to one side. I removed the pads, pushed out the pistons and gave then a good scour with coarse twine dipped in brake fluid, worked them back and fore a few times almost immediately feeling an improvement. After that there was only barely detectable pulling under heavy braking and not holding the wheel, and it passed the retest easily.
|Ah, I see you quoted a price from Burlen in Euros so I guess Eire.|
|Yeah Im in Dublin. I wasnt overly interested in emmissions for the NCT as I havent put the car through the NCT. I was going by what the CO level should be when the engine is tuned properly. it said 3%, so thats what I set it to. Our fail limit for the NCT is 4.5 as well, but dont forget, classic cars in Ireland dont have to do the NCT once theyre over 30 years old.|
|Now that's Irish for you - the older the car gets the less it gets tested :o) It reminds me of the situation with learner drivers (since rescinded I believe) that once they had failed their test twice they no longer needed to be accompanied! That and the distances in kilometeres, and the speed limits in MPH.|
But what says it should be 3%? That would be rather weak even for a late model MGB, if it doesn't have to be tested you will get the best performance and best economy if you adjust the mixture for the highest idle speed.
|Yeah the new legislature regarding licenses now means that what was once a provisional license is now called a learners permit and a permit holder must have a fully licensed and insured driver capable of driving the vehicle with them at all times. That only changed yesterday in fact. The way it used to work is a provisional license used to last a year. When one reapplied to get a replacement, the restriction that stipulated they needed to have a "co-driver" was lifted and they could drive on their own. This meant that people could drive ad-infinitum without ever passing their test at all. It was lunacy really. |
As for the NCT, motor homes and classic cars, amongst other things, dont need to do it.
I think I got the 3% figure from the haynes manual. It runs well at that so I think I'll stick with it.
|The handbook that came with my car (1980 GT) says 3% CO at tickover.|
|Excellent. Thats how I set mine, and Im sure my 78 GT is the same.|
|That *is* the rubber bumper UK engine 18V846/847 i.e. 74 1/2 to 80 but I don't know why they specify a tighter limit than the MOT. I think it is a misprint as all the other engine numbers specified relate to North America which had emissions limits from 67, whereas UK engines didn't have to meet emissions limits until August 75. I've just setup a 78 to pass the MOT, set to the lifting pin it came up as about 4.5% on my Gastester so I weakened it as a precaution by 1/8th turn and on the test it came in at 3.9%.|
|The reason stated is because when the CO level is as it should be, ie: 3% for the later B series engines, it means the stoichiometric ratio is also right. Each engine has an optimum CO percentage based on what it would be if the stoichiometric ratio is also right. So if you have yours set to 4.5% CO, it means youre running rich, and there's unburnt fuel coming out the exhaust. Anything over 2% in fact is technically unburnt fuel, but based on the combustion cycle of the engine, a certain amount is normal, and setting the fuelling so its getting more than the percentage stated as optimum by the manufacturer means that fuel is just being wasted and being blown out the exhaust. Thats what the machine measures. Using the gastester along with my colortune verifies this for me.|
|No, I didn't set it to 4.5%, I set it to the most efficient and effective mixture by using the lifting pins as stated in the Workshop Manuals, then *checked* it with the Gastester, and slightly detuned it to be sure of passing the MOT. You can't use the CO test to setup the carbs as you can't differentiate between them and you must get the mixtures balanced as well as the airflow.|
The 2% on the gastester is simply a calibration point. The gastester doesn't mention CO it measures something else, which varies like CO does at around these figures, and exists at '2%' in free air. Modern engines are much more efficient than engines of the MGB era and typically emit 0% CO and 0 hydrocarbons these days. You can't adjust a 70s engine to modern limits just so as you aren't emitting unburnt fuel or it will be very weak which will burn valves and run too hot.
|Take a look at this link...|
Its the manual for the gastester, which outlines exactly what it does and how it does it. If what you're saying is true, then this machine is absolutely pointless. I think the results with my engine speak for themselves. All my spark plugs are a light brown and clean and the engine is running well. I kept the carbs balanced and adjusted both carbs equally, ie: If I turned one a quarter turn, Id do the same to the other. I did use the lifting pins as well to make sure one carb wasnt too rich and the other too lean though, so I used a combination of a lot of things.
|"(NB: The calibrate position represents what the instrument should register when the probe is in the air. It is coincidental that air should measure the same as exhaust gas with 2% CO)"|
It's not pointless, but it is only a guide and doesn't measure true CO like a garage machine does.
|Ah well I know it's not as accurate as a €4000 krypton machine, but I like what it does. Im delighted about how my car is running now.|
This thread was discussed between 22/06/2008 and 04/07/2008
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