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post #3 of (permalink) Old 03-03-2004, 09:58 PM
highfill
 
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Re: distributer vaccuum advance question

You are running into the issue of ported vs manifold vacuum. Off the side of the carb is ported. Do some searches for "ported", "manifold", and "advance". Neither one is always right or wrong. Some engines use one, some another, some a combination of both.

You can also search on my username and will find a bunch of info on this topic. I used this forum extensively in researching how to tune both my distributor and my carburetor.

If I remember right, manifold vac is moderately high at idle, drops off immediately off-idle, then builds with RPM. Ported is closer to nothing at idle and builds with RPM. This would be why your engine changes behaviour... you are changing the amount of advance recieved due to vacuum.
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