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Old 07-23-2010, 11:30 AM
oldbogie oldbogie is offline
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Quote:
Originally Posted by kyle86
Great experiment!!!

I will definitely be swapping out my limter plates.

As for my problem, the tip in ping went away after I increased the jet size another step. Never would have thought it wanted this much fuel.

The only time it pings during cruise now is when I punch it. If I tip in or roll in, it accelerates fine but if I stomp on it, it will chatter. I'm guessing this is an accelerator pump problem and may be able to get tuned out with some cam changes.
You're probably more on the right track with richening the mixture than messing with the timing. Vacuum advance is maximized with high vacuum and minimized with low vacuum. High vacuum forms when the throttle is more closed and lower vacuum forms as the throttle is opened.

Yet you say you're getting detonation as the throttle is initially opened. So the manifold vacuum is going from high to low and the vacuum advance is going from a lot (advanced) to less (retarding). This would tend to show that excessive advance is not the problem, unless you have a very sensitive centrifugal that's coming advanced so quickly with increasing RPM that it puts in too much mechanical advance too soon. Could be, but I doubt it.

This sounds like the mixture is falling lean when the throttle is opened. When the throttle blades open the mixture flow from the venturis all the way to the valve slows as the greater available area thru the throttle plates allows the mixture to expand, this reduces velocity. The engine responds to this situation with a stumble that is partly caused by a momentary leaning of the mixture and a drop in the inertia of the mixture which is necessary to overcome the reverse pumping of the rising piston when the intake is still open. The accelerating pump on a carb is specifically there to at least overcome the momentary leaning of the mixture until the main metering can catch up.

I don't remember your intake configuration which has a major influence upon this event. Cold intakes on the street with high gearing and an automatic tranny make this a bigger problem. Exhaust heated intakes, as the OEM makes these things, have less of a problem because the hot spot under the plenum keeps the fuel evaporating into a "perfect gas" (as defined in your Physics book) which keeps the mixture closer to proper ratios without having to go to bigger and bigger jets or more and more accelerating pump stroke to smooth these throttle transitions. so much for the idea that cold intakes ipso-facto make more power, yes, but under very controlled conditions seldom seen on the street.

You have to find the music the carb and ignition will dance to. This can be that much larger jets with more and faster advance will run better and provide more fuel economy than leaner jetting with less advance. You have to play with these combinations rather than follow somebody's rules. The rules just get the motor running and provide comfort to minds that don't understand the science under these things. When you hit tuning problems, you have to experiment and to do that successfully you need to understand how the pieces play with and against each other.

Bogie
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