Re: complicated tuning with performance carbs.

ken gilbert /
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one of the finer points of carb tuning i see overlooked is how the various circuits come into play at different operating points. by operating points, i mean RPM, or more specifically, mass intake air flow rates.

for example, most tuning guides/advice tell you that the main jet is working at 75-100% throttle. however, that is not always the case. the main jet is really what clamps the MAX fuel flow rate. at low rpms and WOT, the main jet isn't really restricting fuel flow at all, because you are not delivering the max fuel rate (which occurs at max power). the better definition of where the main jet is important is WOT *and* high rpm.

that means that during low RPM + WOT conditions the needle is dictating how much fuel can be drawn from the atomizer into the throat of the carb. you can verify this by removing the main jet itself--if the needle is correct, the low/midrange will be fine even with the throttle open all the way. as the RPMs climb, the pressure drop through the venturi reaches a point where too much fuel will be sucked up, and it will bog richer and richer with the higher intake air flow rates.

drop the needle a few slots or go to a shallower angle needle and the "too rich" rpm will go up since the orifice size has decreased. the opposite is also true (raised needle or steeper angle, bogs rich @ lower rpm).

i've found the best success in needle choice/position when i start to rich bog right around where the pipe comes up and i've got an oversized (or no) main jet installed. then you clear up peak power and up by reducing the main.

put another way the main jet is a sort of "clamp" on how much fuel can be be pulled into the atomizer no matter how much vacuum there is in the throat. i say clamp but of course it's not a hard knee.

basically WOT tuning has ranges, and the lower range is dictated by the needle and needle jet, and the upper range by the main jet.

just some food for thought.

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