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MG MGB Technical - vacuum gauge readings
| Recently rebuilt engine, rebore, new pistons, Peter Burgess fast road head, BP270 cam. Just 250 miles on it so far so not run in yet. I've not been too happy with how it's running. I attached a vacuum gauge this afternoon and am not sure how to interpret the results. At low idle speed (600 -800 rpm)the gauge is a blur going from near zero to over 25 (full scale on my gauge) as the speed picks up this band reduces to 15 to 18 (at 2000rpm). I know it doesn't look good but where should I start looking. Payen gasket torqued correctly. When I checked the compression shortly after putting the engine in all cylinders were within 5 psi of each other. I'll check again. This confirms my feeling that something isn't quite right. Any ideas what to attack first? |
| Steve Church |
| Steve, where on the manifold did you put the pick-up point for the gauge? I'm assuming you've balanced the carb's correctly and have the mixture about right? Bouncing around that much at idle suggests an air leak at the manifold gaskets as a start point but to be honest, with the amount of tuning done, you really need to get it to Peter. MGmike |
| M McAndrew |
| I came to the same conclusion after reading up on vacuum gauge diagnosis, so I have just removed the carbs, installed new gaskets coated with high melting point grease and checked the manifold nuts. Not the slightest bit of difference. I have set the jets to 65 thou down and done the 'lift' test but reckon I'm not experienced enough yet but I think the mixture is somewhere near right having looked at the plugs. Both carbs are set the same. I have taken the vacuum gauge off the base of the breather valve. I epoxied in a small tube. I don't have any easystart so can't do any spray testing yet. I was planning a trip to see Peter when I have sufficient miles on the engine and also making sure that I have an efficient ignition system (electronic points in original dissy). I have a few more hundred miles to do yet to get the engine run in. In the mean time I will check compression and valve clearances. |
| Steve Church |
| Ahh, breather valve! Earlier than mine with the crank breathing via the valve in the manifold and not directly in to the carbs? Start there.... First thing to try is to open the valve and take the spring out put the valve back in and the spring on the outside of the diaphragm to hold it closed (you can worry about crank breathing later!). You should now have a very stable vacuum at idle. If so change the diaphragm and check the lid of the valve has a small hole in it. If the valve is failing you have a good chance of setting the mixture a little week. If this works and you want the full technical explanation i'm sure Paul H or FRM will oblige ;o) Personally I would get a set of later carbs and put the breather pipes direct. After all, there was a reason for the change!!! good luck. MGmike |
| M McAndrew |
| Steve, what rocker cover is fitted and does it have a vented cap? ~If not you'll need to fit one or remove the cap whilst running with the valve closed. Are you getting much crankcase pressure with the cap off eg blowing out the rocker cover? MGmike |
| M McAndrew |
| The cam will cause lower and possible erratic vacuum at low idle, but I do not have experience with the precise setup. The cam could be affecting the Smith's valve as well, with pulsations. A possibility there is to fit a restrictor at the bottom of the valve, as a damper. Or the pulsations may be resonant at that take off point, and affecting your gauge. If your manifold does not have sundry holes or bosses with plugs or fittings - most do - then drill a hole away from that rather (acoustically) strange area. Any 1/2 head 5/16UNF bolts on the top of the manifold are plugs suitable for vac gauge, and later cars use the appropriate adaptor for dist adv takeoff, or you can drill a bolt and make one. Are you certain the cam timing is correct? Sticky valves can do it too - but if it is Peter's head I'm sure that is right. Suggest you set idle up to 1000-1200 and see what happens - there are reasons that production cars do not have cams like this! The Spitfire I am working on has an even hotter cam (or did until it died), and idles like a kitten at 1300 with a Smith's valve. While I agree that the direct to carb vents are better, mostly for maintenance reasons, there is no good reason or great benefit to changing carbs. And, at 250miles it should be pretty well run in if it was built and treated correctly - at that mileage I'd be scaring passengers and having fun! (But not running on Motorways) FRM |
| FR Millmore |
| Steve, I wouldn't worry too much. It may simply be that you need more damping of the vacuum. I have used vacuum gauges on my Magnette engines for more than 40 years and without damping they are virtually useless, as the needle flickers all over the place. Some vac gauges may have damping restrictors built in, but the ones I have used such as the old Redex gauge used a simple screw clamp on the vac hose to adjust the damping. Clamping is not the ideal method as the compression of the tube varies with ambient temperature changes. However, you can also buy carb spacers with built-in connectors for vac gauges and these have very small orifices to provide some damping. (See Photo) I think this link is the right one: http://www.minispeed.co.uk/content/carb-spacer-vacuum-gauge-take-75mm-hs4 To begin with you could try a screw clamp such as this one: http://www.thesciencefair.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=3440-14&Category_Code=lab-cla and see how you get on. Andy  |
| Andrew Dear |
| Andy, My gauge is a Redex one. I thought the clamp was just to help attach the tubing. I'll try tightening it in stages. I will also move my vacuum take off point to a bolt on the manifold (there are three!! I'll drill it out and epoxy the pipe in. I'll also try removing the breather valve. I did examine the rubber diaphram and could see no splits. FRM, I was aiming for 500 miles at least. I've never had to run in an engine of this vintage and thought you needed loads of miles of treating it 'sensibly'. |
| Steve Church |
| Steve- Long run in was because machining and bearings were not very accurate. That pretty well ended by 1950 for most manufacturers, but the tradition lived on. In fact, the worst thing for an engine that was accurately machined and assembled is to baby it at first. In particular the first half hour for cam & tappets, and the first 50 miles for rings and bores. The next bad thing is constant speed/load use, hence my comment about motorways. Vigorous country twisties is the ticket! Head retorque, valve adjust, and oil/filter at 500. (You should already have done the head at least twice) FRM |
| FR Millmore |
| FRM, Definitely no motorways around here. All twisty roads. I ran the engine at 2500 rpm for the first 20-25 minutes to bed in the cam.. I have not retorqued the head yet. I was aiming to do that at 500 miles (didn't know I had to do it before then). To retorque, is it just tightening up, or do you slack off half a tuirn then torque back up? And is it one at a time? I have a 15 mile trip to a festival in about half an hour. I hope a retorque is not critical!! |
| Steve Church |
| Hi Steve We do not retorque Payen composite gaskets. Copper gaskets must be retorqued. Slacken and retorque each nut in turn. Best bedding in is at peak torque when the ring pressures are greatest. This means using the engine at 4000 ish spritedly. But not seeing how fast car goes in third and fourth and not making it slog uphills in a high gear. Enjoy your festival. Without some gauges we would not worry when we do not need to. Keith, my Stepson, fitted a temp gauge to his VW bus. Tuned engine will run hotter. He was alarmed at readings. 5 years on and engine still flying. Why worry? All the armchair internet experts worried him to death. None of them said what did the gauge read on his previous engine, to which he would have said only fitted on new one so no clue ! Peter |
| Peter Burgess Tuning |
| Steve/Peter- Despite Payen (and other "no retorque)claims, I still retorque as soon as it is hot, after a dozen heat/cool cycles, and at 500ish then about once a year, or two years after the first two. I have measured movement in nuts on Payen, but not as much as on other gaskets. And I have replaced Payen fitted by others and not retorqued. This may have changed in recent years, since gasket technology is constantly changing. I have even cured severe oil leaks by retorquing the Mazda MLS gaskets at 130,000, so I expect I dodged an imminent HG failure = 10 minutes and no $$. Mark the studs and nuts, back off about a 1/4 turn and pull smoothly to torque - if the nut is in the same place, it is stable and you can extend or eliminate further retorques. FRM |
| FR Millmore |
| Peter, Thanks for your comments. I drove the car a bit more spiritedly yesterday. It confirms my thoughts that something is'nt right. Absolutely no 'zip' at all. In fact I believe the car went better before I rebuilt the engine. So something is'nt quite right. I have checked the timing and carbs (plugs look ok, exhaust pipe perhaps a bit darker than I would like). I'm rechecking the valve clearances tonight (reset them about a month ago. I installed an electronic points set (reset timing after). I rechecked the compression figures last night and no.4 is down 15 to 20 psi on what it was originally (checked after I had run the cam in). 1 to 3 are 175 to 185, 4 is 165. Not much of a difference but 4 has changed and 1 to 3 are unchanged. I'm probably being concerned unnecessarily but at the moment I'm just a bit disappointed (probably just Monday morning!!). |
| Steve Church |
| Steve- Recheck cam timing, best chance for error in a rebuild - runs like a dead marshmallow = late cam. You should try one with a sheared crank key and the crank pulley bolt welded to keep it from unscrewing! (AH 3000, about 90 deg late before the debris wedged the sprocket) Check compression in reverse order, as battery drain can give that kind of drop if you go 1>>4; or have the charger on while you do it. FRM |
| FR Millmore |
| I did take particular care when doing the cam timing as I needed an offset key. However if my method was wrong..... Can I measure the timing accurately with everything assembled? And, if I have to, is it possible to reset the cam timing with the engine in the car (with rad removed)? |
| Steve Church |
| Measure #1 inlet peak opening and compare with your cam card. If it is a symmetrical cam, then overlap on 4 = TDC on #1. If not, you can calculate the equivalent position at overlap equality. I check like this, setting #4 valves at about .045. then using feeler gauges for even clearance. Don't forget to reset - scares the bejeezus out of you if you start it with two valves at .045. If you can get the bolt and damper off, should be no problem to change. Jack up engine or unbolt rack. FRM |
| FR Millmore |
| Thanks FRM, but I didn't follow any of that. It's a case of 'if you've done it before you know what I mean'. Phrases like 'overlap on 4 = TDC on #1' need further explaining. I only know the method using a degree wheel and dial gauge which I suspect could be a little more difficult with the engine in the car (I pasted a printed degree wheel pattern onto plywood and cut it to fit over the pulley(damper) - could be a problem with the cross member) |
| Steve Church |
| Hi Steve Rule of thumb test. At tdc the inlet and ex valves should both be open approx the same amount. rotate the engine back and forth until the in and ex valve lift looks similar then see how far off tdc the timing mark is. With the HR270 inlet opens 28 degrees afterTDC and ex opens 28 degrees afterTDC so lift at tdc is even. Did you check ignition timing at higher rpms with no vac? Peter |
| Peter Burgess Tuning |
| Hwat is corect ignition timing at higher rpms. with no vac.? 18v engine vidth Peter Burgess fastroad head std. cam. Maniflow exhaust Harry |
| HL Lagoni |
| Steve- As Peter said. More accurately as I said. Since Peter has confirmed this as a symmetric cam: If you set #4 valves to some setting which gives around .010 clearance at overlap, you can then accurately measure the clearances with thin feeler gauges - which you can "feel"! - how about that? On most cams this will be about .045-,050 in the normal setting procedure. When the clearances are equal, the cam is where it should be for TDC #1 firing, just look at the mark. Hopefully you marked the pulley when you had the degree wheel set up - if not, well, calculate and mark off the pulley, or interpolate from factory marks & pointers. You can calculate and print a tape to wrap around the pulley if you like computers and have an accurate printer. Or, by the rules of geometry for constructing a hexagon, using only dividers, you can set dividers at the radius, and mark off 60 deg either side of the TDC mark, then subdivide to any desired degree of accuracy. Peter - did you get my confirmation re the Spit cam? Email has been strange. FRM |
| FR Millmore |
| Hi Fletcher Email arrived safe and sound, just waiting to get some clear space time to ring Piper. Harry try 32 degrees btdc at 4000 rpm. Peter |
| Peter Burgess Tuning |
| Also, check your distributor cap for arcing on the inside. This can occur when changing over to "electronic points" as they don't always line up exactly, on the point mounting plate, as the original points did. When this occurs, the rotor and cap contacts wind up being quite far apart and this can cause problems with the ignition system. RAY |
| rjm RAY |
| Hi Peter OK, thanks much, we are on edge about not getting this ever! But not a real hurry as long as we know it is coming. Steve - what Ray mentions is called "phasing" of the breaker trigger to the rotor and cap terminal positions. There should be instructions about this in any conversion kit where it is an issue. It's why some rotors have long and/or offset "contacts". FRM |
| FR Millmore |
| I checked the valve lift at TDC for #1 as I had the engine at TDC firing for #4. The values were axactly the same. I tried setting the #4 clearances so that I had about 10 thou clearance at TDC (as per FRM's instructions) but I did not have enough feeler gauges to set the initial clearance. I ran out at 90 thou. So this checks out roughly as per Peters rule of thumb. I shall probably check more accurately with a degree wheel when I have a bit more time. I also checked the valve lift on all the valves in case I had stuffed up the cam at bed in time. All came within 10 thou of the manufacturers figure so I recon the cam is ok Peter, is the 18 thou clearance you suggested ok? Piper quote .014/.016 on their website. I need to button up the rocker cover now as I have the Norwich 'Cathedral Run' to do on saturday. A day visiting a lot of local churches during churches open week. |
| Steve Church |
| Steve I think you had the HR270 which is 16/18 for the version we supplied you. Peter |
| Peter Burgess Tuning |
| Peter, A couple of questions. 1.Are the two figures (16/18) the inlet/exhaust settings or the hot/cold figures? 2.Why are these different to the 15 thou of the standard cam. 3. What's the difference between the BP270 and HR270? 4. I used the valve lift figures for the BP270 of 0.405". Is this the same for the HR270? |
| Steve Church |
| HR270 nominally 0.397" lift. Tappet settings vary with cam shape and are suited to a particular cam. We use the 16/18 as cold settings, 16 in and 18 ex. BP270 will vary depending what is current for Piper, we lock down the spec by using particular profiles ground to LCAs we choose. I reckon you could do with a trip to the rollers. Did you check your ignition advance without vac at different rpms? Peter |
| Peter Burgess Tuning |
| Peter, Thanks for that. We are planning a long weekend in Derbyshire when I am sure that the car is as fit as it can be, i.e. making sure I haven't done anything stupid when I assembled the engine. I feel that something is not quite right that I should be able to sort before I bring it over to you. I'm sure you have enough work on your hands without sorting out other people's mistakes!! I've also only 300 miles on the engine so far. Is this enough running in before a RR session? |
| Steve Church |
| I am sure you have done the work ok, getting the ignition and fuelling correct makes a significant difference in many cases. I know you worked very hard sorting the cam timing. 300 miles should be sufficient. The inertia rollers we use do not stress the engine. We only load the engine to check for pinking when we have finished tuning. Peter |
| Peter Burgess Tuning |
| Peter, I checked the ignition timing tonight. Vacuum disconnected. 800 rpm 14 deg 2000 rpm 20 deg 3000 rpm 26 deg 4000 rpm 28 deg Didnt go further than that as I was doing all if it my self. No attractive assistant. She was watching the tennis!! |
| Steve Church |
| Steve, checking Paul H's site you need to move it out another 8 deg advance. should be 800 rpm 10 static + 8 centrifugal =18 max at 2200 10 + 20 =30 possibly less one or two for unleaded. MGmike |
| M McAndrew |
| oops should have said an extra 4 deg! Number got a bit jumbled.... MGmike |
| M McAndrew |
| MGmike - you figures only apply to the earlier dizzys which maxed out at 2200 rpm. The 41234 or 41391 went on advancing up to 6000 rpm and from Steve's figures that looks like what he has. Strobe timing should be 11 BTDC at 1000 rpm for the later dizzys or 13/15 BTDC for the earlier ones. If setting the idle strobe results in higher advances at higher rpms then the dizzy is knackered! If indeed the dizzy is a 41234 or 41391, Steve has too much advance and advancing another 4 degrees is the last thing he needs to do! Just another observation - in my experience these engines run best on the rich side - I'd be looking for a CO reading near 5% - a definite small rise in revs when the piston pin is lifted. |
| Chris at Octarine Services |
| Chris, point taken. Looks like I've fallen for the ASSuME as Steve has a late sixties motor ;0) Steve, confirm dizzy and adjust accordingly. MGmike |
| M McAndrew |
| The dizzy number is 41155 B. Engine is a 18GD series. Car is Sept 1968 build. None of the numbers mentioned so not sure where to set! |
| Steve Church |
| Steve, is your car a re-import? From Paul's site the 41155 is for a USA and Canada of the same age. But it could be just the only one left in the bucket at the time.... 10 static 20 at 1000rpm and keeps going well up to 30 at 3k plus vacuum! Me thinks a new dizzy is called for with a set of weights to match your engine. It seems a shame to destroy all your good work for the want of a 123.... MGmike |
| M McAndrew |
| Car definitely not an import. Originally sold to a UK dealer. I've no idea if this was an original fit. Yes I was wondering about the 123 Tune and how easy it is to optimise (I fitted a Megajolt to my midget but never got it set optimally). Assuming that is, that one of the curves of the standard 123 isn't suitable. Peter if you are viewing what's your opinion on this? Standard 123 or 123 Tune for a modified engine? |
| Steve Church |
| Steve, no brainer... 123Tune set it up with a standard curve and get Peter to tweak the numbers from the RR session. I've not be looking to buy a dizzy yet but its added to the shopping list. MGmike |
| M McAndrew |
This thread was discussed between 28/07/2012 and 04/08/2012
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