Possible cause? Advice, please.

[wilkin250r]

Originally Posted by willett

thanks, that makes sense.



this is sort of off topic, but in my quest for knowlege I came across this article about timing:

http://www.chevytalk.org/fusionbb/sh...hp?tid/239321/

That's a pretty good article, you'd probably have to read it a few times in order to digest all of it. I don't know if I agree with the "ported vs manifold vacuum" 100%, I think there are a few other aspects to consider.

The one aspect that I DIDN'T see in that article, however, was the necessity for the mechanical advance. It did a good job of explaining the vacuum advance because of the lean mixture at a idle needing more time to burn, but it didn't go into an explanation of why the mechanical advance is needed.

It's rather simple, really, and probably why the article didn't explain it. It's simply a matter of speed. The combustion process takes TIME to burn, not degrees to burn. Let's pretend you want to spark 75 milliseconds early, that translates to 12 degrees at 1000rpm. But think about it, at 2000 rpm, the motor is turning twice as fast, isn't it? So 12 degrees is only 37 milliseconds, you need 24 degrees to get back to your 75 milliseconds. And at 4000 rpm, you would need 48 degrees.

As the engine is turning faster and faster, each degree of revolution takes less time. So you'll need more and more degrees to reach the same length of time.

This is an over-simplification, of course, but that's the general idea.