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September 29th, 2008

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Q & A: Maximum Advance

Friday July 06th, 2007
Q & A, MGA, MGB, MGC, MG T-Type, Midget
Hi John:



I have spoken with you several times over the phone over the past many years and you have always given sound advice. You have helped me with distributor questions in the past, as well as a problem I had with a hesitation at
3300 - 3600 rpm, and I (we) have made progress over the years in making my "73 MGB run better. I have had to replace my distributor a couple of times over the years before I knew that the bushing could be replaced and the
original distributor rebuilt. I currently have a 25D and 45D that will both run in my car (neither are correct for my year of MGB or may not even have been for an MGB). I have had both tested on a distributor machine and the
25D has 28 degrees of mechanical advance at 4000 rpm and the 45D has 22 degrees at 2200 rpm. I have used 32-34 degrees as a maximum advance (as per your advice as well as published in Peter Burgess's book) and have set the
static accordingly not to exceed this value. Manifold vacuum is attached and functioning, but only plays a roll at lower rpm and is a non-issue at higher rpm, therefore max advance should not be exceeded, as I understand
it. The car seems to run better with the 45D. The car has a mild cam, some head and port mods as per instructions from Peter Burgess and the compression ratio is about 8.7:1.

I was looking through some old notes and correspondence last night and found a note I had written to you many years ago asking similar questions and in your reply you said, "...you need 20 -25 degrees at idle but maximum advance by 3000 -3500 rpm is 45 degrees." My question now is which value is the maximum advance that should not be exceeded, 32 - 34 degrees or 45 degrees?

Thanks for your help.

Cheers, Rob
Rob,

My old figures were wrong. The proper advance for the distributor is 32 MAX, vacuum disconnected. That 45D distributor will give you a better advance at the lower speeds.

Hope this helps!

Joh... read more