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post #12 of (permalink) Old 05-26-2005, 05:14 PM
RRich
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Join Date: Sep 1999
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Re: vacuum advance question

IF you use manifold vacuum: You said the ported vacuum comes in too fast and too strong. Maifold vacuum is there ALL the time! Isn't that too fast?
The only time it drops is when you are under heavy load - so does ported vacuum.

Sooo - if you use manifold vacuum it's advanced ALL the time. At cranking there isn't enough to feel, but once it starts it "sees" initial, about 8, plus the 15 or so from the vacuum advance = 23 or so. Ping city.

Try it.

Now to get it better (less worse) - leave the manifold vacuum connected - so it's advanced fully - then set initial to the 8 while it's still connected. (What that does is negate the manifold vacuum advance.)

Now as you drive it - only the mechanical does any advancing -- it'll add about 15 to the initial - giving you about 23 at cruise. Then, when you punch it or get under load, the manifold vacuum will drop, dropping timing clear back to initial - about 8! Not much power down there.
And it'll be hard to start because during cranking the manifold vacuum isn't enough to pull the advance up - so it'll be trying to start at 8-15= 7 degrees retarded. Not easy to start down there.

But - don't take my word -- TRY IT!
Most people don't understand what an engine "needs" and what the advance really does - or how it works. Lots of bad advice around.

Try those things - you'll see.

Best is get the right adjustable - or get creative - install /adapt one from a GM. Sometimes all it takes is redrilling a couple of holes.

Let us know.

Doubtful the lack of a cat reduces backpressure enough so EGR valve doesn't open. The EGR should also be running on that ported vacuum you have. The EGR will only open at higher R's anyway. To test - stuff a wet rag - wet so it doesn't catch fire - in the tailpipe - rev it - it should open.

But the trouble you are experincing is exactly as you stated in your first post - too much too soon advance.
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