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MG MGA - Slow or fast timing – which takes precedence

On getting the car back from an engine rebuild some time ago it ran well but occasionally when feathering the throttle at cruising speed the engine will stutter/shake. By revving this clears instantly and runs fine. Under hard driving the engine runs perfectly, pulling strongly to cruising speed. Having gone through carb rebuild and more recently fitting an electronic SU pump the occasional rough running is still there but only ever on light throttle. Having thought about this for some time and going through the archives here I had decided this is probably linked with vacuum advance and that the timing was probably too advanced, so with the vacuum advance kicking in at cruising speed the timing was going too far advanced. Today I checked the timing and to my surprise at fast idle the timing was barely 10 deg BTDC. I probably should have checked the timing at 3500 rpm without changing anything but didn’t. I reset the idle timing to 20 deg BTDC at close to 1000 rpm and then checked this setting at 3500 rpm without vacuum – the reading was 40 deg BTDC! So I have the following questions

1. Is my diagnosis likely to be correct that the Vacuum Advance at light throttle is causing the stutter – timing too advanced.
2. Should I retard the timing based on 3500 rpm to 32 deg instead of maintaining the idle timing
3. Will this retardation proportionally reduce the idle setting eg back to 12 deg
4. The distributor is a DM2 40510A with a lumenition module, is it time to replace – have read about the Chinese built dizzys supplied by Bob West, should I look at these or the 123 Electronic system.

Thanks Kevin
K Garner

Not sure who in the archives would ever suggest that this was a timing issue??

Please put your timing to 32/3 degrees at ull max mech advance more than say 4500 RPM then the timing is spot on irrespective of dissy type or condition

Your problem is CLASSIC weak mixture, it is possible to be other things but first set your carbs up. If you are not competent then simply enrichen them by 2 or 3 flats to see the effect.
Robert (Bob) Midget Turbo

As Bob said, most likely weak mixture in mid range. It may be weak mixture throughout the speed range, including at idle, in which case a standard tune up would fix it. But do adjustments in the correct sequence.

Set timing first. Using a set-back timing light, set timing at 32d BTDC at 3600 rpm (or any higher speed) with vacuum advance DISCONNECTED. Setting timing at that point can can compensate for a multitude of problems with both vacuum and mechanical advance components. (The rest of diagnosing vacuum and mechanical advance problems is for a different discussion). Reconnect vacuum.

Then synchronize air flow for the carbs. Very simple and not particularly critical, 1/2 turn open for each idle screw, and then lock the shafts together.

Then adjust idle speed to 1000 rpm (or slightly less). For the follow up mixture adjustment you need to keep the idle speed under 1000 rpm.

Then adjust mixture at idle speed, by the book, using the lift-the-pin method to judge proper idle mixture on each carb.

Then test drive it. If the hesitation on light throttle in mid range persists, then enrich the mixture one or two flats on each carb and test drive again. If you have to go 3 flats or more to make it run well in mid range, then it's time to check the numbers on the needles to see if you might be running the wrong needles (being too lean in mid range). If the needles are wrong, then getting it to run well in mid range may leave it running too rich at low speed.

A worn main jet can cause similar symptoms. If the jet had previously been off center so the needle would drag on one side at low speed, then the jet orifice would be worn oversize, and the needle may have a flat worn on one side. If you then raise the jet substantially to achieve proper mixture at idle speed, it will run lean at higher speeds.

Top check for wear in the jet, insert an 0.090" diameter drill rod and see if it is snug or will wiggle. To check for wear on the the needle requires careful measurement with a micrometer to see if the needle is out of round, especially within the first three steps at the fat end (idle range on the needle).

Always do the diagnostic work first, before you throw money at parts it may not need.
Barney Gaylord

Just tuned my own car up and went out for a drive the other day - engine seemed to be lugging, if not quite stuttering and shaking - just after upshifting during acceleration - esp. from 2nd to 3rd.

Works better if you step on it!

I richened the mixture one flat on each carb, and it now sounds and acts a bit better.

This is an easy thing to test, though it can be difficult to reach your hands up under the carbs.

I think I might go one more flat on each carb and see the result - right now the plugs look tan and even, though, so it's probably spot on right now.

It'll be interesting to check my gas mileage this year. Last year it was terrible, and the car always ran rich.

JIM in NH
AJ Mail

Thanks guys, I will set timing as advised at 32d 3600rpm and richen mixture. Given that the carbs were recently refurbed by BoB West I assume needles etc will be fine, however, when when the engine was rebuilt the cylinder head went to Peter Burgess for his Econotune lead free conversion and flow work. Might I need to look at a richer needle profile as at the moment the needles are standard GS. Incidentally the car is a 1957 with original 1500 engine.

Thanks Kevin

K Garner

You won't know until you try it, but if the head has nee ported for better flow you may need a slightly richer needle, like the stock 1622 engine needle for instance.
Barney Gaylord

Further to my question on timing, as recommended I retarded the timing by 8d from 40d to 32d at 3600 rpm with vacuum disconnected. Refitting the vacuum and road testing the car ran and pulled well, I tried feathering the throttle at cruising speed to see if I could get the engine stutter I previously reported but nothing apparent - though I could go days without this occurring previously so time will tell. Back at the garage I checked to see what the dynamic timing was at 1000rpm and this turned out to be 12d BTDC, prior to setting the timing at high speed this had been 20d. In other words retarding by 8d at 3600rpm (vacuum disconnected) resulted in 8d reduction at 1000rpm (vacuum connected). Surely with the mechanical advance working properly I shouldn't be seeing this linear result? Should I be looking to replace/refurb the DM2 distributor.

Incidentally and going back to my initial posting, using the lift pin method the mixture on the newly refurbed carbs was if anything slightly rich and for the time being I haven't changed this (not enough courage to mess).

Thanks Kevin....
K Garner

Kevin, when you adjusted the tming by rotating the dizzy, that adjustment applies at all rpm.
Art Pearse

Kevin

Lets look at this logically

The reason for mechanical advance is simple. When we ignite the sparkplug the mixture requires TIME to fully detonate and thus we generally ignite the plug slightly in advance of the piston being at TDC. So we could have a figure of say 10D BTDC During this period before TDC the mixtures comes up to fully ignited and ready for maximum pressure.

Now the length of time required remains the same however if the engine is spinning quicker (More RPM) then the time available reduces and thus the spark has to be triggered earlier to compensate as RPM rises.

Typically then we have say 24d total mechanical advance and so we need to set the timing at 8 or 10 d BTDC at static so that at maximum revs we will see around 24 = 8/10 to give us 32/34 degrees full mech advance and generally this will be linear.

On top of this we have a further variable that of vac advance. This attempts to advance the timing dependant upon throttle opening and suction. This advances the timing when we have partial throttle opening, because with less mixture the time taken to fully come upto pressure is more and so we need to advance the timing to keep the engine running at max efficiency and so fuel economy is improved by this technique. Without vac advance the engine would perform perfectly but fuel economy would suffer.

For your purposes timing should always be set with vac DISCONNECTED.

Hope this helps.
Bob Turbo Midget England

Art/Bob, thanks for responding and yes I now see the obvious logic. Apart from educating me you have probably saved me in further unnecessary expense.

Kevin
K Garner

This thread was discussed between 19/05/2012 and 28/05/2012

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