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MG MGA - Vacuum advance

What is the vacuum unit specs stamped on the unit ( 3 numbers) ?
Art Pearse

Art. The DM2 Lucas distributor, specification number 40510H, original to my 1961 MGA shows a mechanical advance cam stamped with a 12, indicating twelve degrees of mechanical advance observed as 24 degrees of mechanical advance at the crankshaft.

The vacuum advance unit has the threaded end which was continued into early MGB production on the Lucas 25D distributor. The markings on the MGA unit are 7-14-10 indicating that the vacuum advance begins at a vacuum of 7 inches of mercury and is fully applied at a vacuum of 14 inches of mercury with a total of ten degrees of additional advance supplied. (I seem to remember that this was degrees of advance observed on a distributor test machine, but cannot locate an immediate source for this belief.)

I believe the vacuum advance characteristics are quite similar to the early MGB units. The original MGB distributor showed 20 degrees of advance (crankshaft) at 13 inches of mercury. This was for the high compression engine. Thus, it is very likely that an early MGB unit could be used if the proper MGA unit cannot be secured.

As to the mechanical advance curve mentioned on your other post, when the maximum overall advance occurs is governed by the springs used to restrict the movement of the mechanical advance system. Thus, slightly weaker springs would allow the mechanical advance to develop at a lower engine speed.

Les
Les Bengtson

Thank you Les. That is perfect!
Does that mean, that with 10deg static, 24 deg centrifugal (above 3000 rpm) that if you lift the throttle at that speed to give high vacuum, you have a timing of 44 deg BTDC?
Art Pearse

Art. That means exactly as you say. It also means that at a steady rate of cruise, where the vacuum at the carb or intake manifold is above 14 inches of mercury, you get an additional 20 degrees of ignition advance.

When we hear pinging/pinking, the engine is under load and we are opening the throttle--going up a hill or passing someone on the flats. This is when the intake manifold and carb vacuum drops, retarding the vacuum advance. If the mechanical advance is correct--exactly correct or very slightly retarded, and the vacuum advance drops sufficiently (no one has ever calculated the vacuum advance from the first "minimum vacuum reading" the "point of full advance") we do not have any detonation (pinging/pinking) if we are using fuel of adequate octane rating. It all depends on how the system works together.

I have been around her a long time. I have been published in several magazines. Some call me a "guru", a term I dislike in reference to me. But, I will agree that I am fairly competent, have some fairly good training, and have been doing this for over 40 years now. Thus, when I say that I have both the basic knowledge to do this type of thing, and own the necessary machine tools (two lathes, a milling machine, horizontal metal band saw, distributor test machine, et al) to do this work myself and prefer to farm such work out to others, I hope people will listen.

I have done some quite fine second rate distributor rebuilds. The upper tier of second rate from what I hear. Almost first tier. But, John Twist, Jeff S., and, perhaps Joe Curto and Peter Caldwell (World Wide Auto Parts in Wisconsin) do this type of thing on a regular basis. Thus, when I need a rebuild for my own car, I will do it myself. When I need a rebuild for a friend's car, I will help them out if they cannot afford to send the distributor off--and will not charge them for such assistance.

But, for my child (who takes her sports car off for long distances) and for my friends who do the same, I recommend one of the above individuals. They can provide, for a relatively small amount, security and confidence. Something that very few people can do.

Thus, I believe you have most of the basic information you need to make form your ideas and communicate them. All of the people I have mentioned will know who I am. Please contact them, mention that I recommended that you do so, and ask them for their thoughts on what would work best for you in light of current fuels and driving conditions.

Then, do what you feel you need to do. I am fully willing to put my life in my hands. But, I am not always willing to take that responsibility for others. make your decision based on your best thoughts.

Les
Les Bengtson

Les I am sure the distributor cam marking is as you say camshaft degrees but the marking on the vacuum capsule is crankshaft degrees. Denis
Denis4

This thread was discussed between 17/05/2009 and 19/05/2009

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