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MG MGA - Retarded question

MGAs have a vacuum unit that advances the ignition when at high vacuum, and lets it retard to the "centrifugal only" timing when you floor the gas. My son in law is restoring a Tr**mph Sp*tf*r*, with a GT6 engine. Both of these engines have a vacuum system which retards the ignition on high vacuum, in opposition to how MGs work. Can anyone explain this apparent anomaly? What is the theory behind vacuum and timing?
Art Pearse

You're quite right Art, I just checked my midget 1500 (Triumph engine) book and sure enough it says vacuum retard. I don't know, it seems wierd. I bet Barney knows the answer!
Lindsay Sampford

Vacuum retard is a screwy emissions control function. Wanna see how far you can throw it? Vacuum advance generally needs a ported vacuum source tapped into the carburetor venturi. Vacuum retard is usually tapped into the intake manifold. There are some vacuum units with the connector nipple on the opposite side of the diaphragm. When these are tapped into the intake manifold they retard timing at idle. When you step on it and the vacuum drops you get (lack of vacuum) spark advance.
Barney Gaylord

Some TR have both vac advance & retard, coming in under different circumstances - temperature switched ported and/or non ported vac - makes a very strange looking distributor. Can't recall if the double vac ones were Delco or Lucas. UK cars would not have had the double vac, nor do earlier years of 6 cylinder cars. It's all to do with funky attempts to meet the then ever changing US emission regs.
If you are real lucky, Lucas made replacement distributors for the Delco equipped things, possibly THE best replacement part ever! I put a bunch of these in when the Delco ones cut themselves in half - parts for them or complete distributors were impossible to find even when the cars were current.

FRM
FR Millmore

This thread was discussed between 14/11/2009 and 15/11/2009

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