Quote:
Originally posted by bumble
Our gut feel is we need smaller main jets and smaller air correctors but are unsure of the effect of the various emulsion tubes available.
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Mark-
I am no Weber guru, but I am in the exact same boat with my PMO-carb'd 3.4 motor. This seems to be a common pattern with carbs- when the main jets kick in, the A/F ratio goes rich at first, then leans out as the engine RPM increases. I think this is why Porsche changed to the IDS carbs on the 2-liter "S" motors, with an extra high-RPM fuel enrichment circuit.
I would caution against going smaller on the main jets, since that will only lean you out further on the top end. I think the key is in the emulsion tubes, since they are what control your "fuel curve", so to speak. I am trying to find out which ones to try next myself (I am using F2 tubes at the moment), that will give me less fuel delivery at the mid-range and more at the top. Unfortunately, the people with the specific knowledge of these details are few and far between. It is mostly a trial and error deal because of the differences between individual engine specs.
My mechanic has been talking to Richard Parr at PMO, and experienced tuners like Jerry Woods, Bruce Anderson, and Steve Weiner may have an idea or two, but it is not an easy question, and may involve accepting compromises. That is why EFI is used on most modern racing setups- fuel mappings can be optimized precisely throughout the RPM range by modifying the injection pulse.
Good luck,
TT