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General Minitrail Talk
Modifications
special thanks to racerx and Pony Express
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<blockquote data-quote="69ST" data-source="post: 10266" data-attributes="member: 5"><p>When it comes to setting up Mikuni carbs, there are two "wild cards" in the mix: engine displacement and needle jet profile. Horsepower is horsepower, regardless of displacement, however both pilot & main jet sizing is still affected. A larger displacement engine moves more air at idle/off idle and for that reason, pilot jet size is closely related to displacement. That jet/displacement ratio is less clearcut at WOT peak power, but still exists. All else being equal, displacement equals air flow at any given rpm. With less air flow per firing, the smaller engine needs to fire more times (higher rpm) for a given power level. Less air moved per firing requires less fuel (smaller main jet), per firing, to maintain the same A:F ratio. </p><p></p><p>The biggest challenge to pre-jetting is the variation in needle jet profile and different air jet sizes right from Mikuni. The needle jet affects A:F at least as much as jet size. It is possible to run richer with a 120 main jet than a 220, with the same carb, if the needle jet has a larger orifice. The seem to be 2 or 3 different VM26 configurations available from Mikuni. Most seem to require main jets in the #200 or #120 range. This also seems to apply to the VM22 series. Air jets affect midrange metering circuit transitions and are used to tune-out stumbles. None of these parts are readily available, unfortunately, so you're stuck tuning around lean spots with pilot circuit, main jets and needle adjustments only. The point here being that one will have one or the other and the main jet requirements are vastly different.</p><p></p><p>As Jeff has pointed out, an wideband O2 -equipped EGA allows one to quickly figure out what's going on, saving hours of guesswork.</p></blockquote><p></p>
[QUOTE="69ST, post: 10266, member: 5"] When it comes to setting up Mikuni carbs, there are two "wild cards" in the mix: engine displacement and needle jet profile. Horsepower is horsepower, regardless of displacement, however both pilot & main jet sizing is still affected. A larger displacement engine moves more air at idle/off idle and for that reason, pilot jet size is closely related to displacement. That jet/displacement ratio is less clearcut at WOT peak power, but still exists. All else being equal, displacement equals air flow at any given rpm. With less air flow per firing, the smaller engine needs to fire more times (higher rpm) for a given power level. Less air moved per firing requires less fuel (smaller main jet), per firing, to maintain the same A:F ratio. The biggest challenge to pre-jetting is the variation in needle jet profile and different air jet sizes right from Mikuni. The needle jet affects A:F at least as much as jet size. It is possible to run richer with a 120 main jet than a 220, with the same carb, if the needle jet has a larger orifice. The seem to be 2 or 3 different VM26 configurations available from Mikuni. Most seem to require main jets in the #200 or #120 range. This also seems to apply to the VM22 series. Air jets affect midrange metering circuit transitions and are used to tune-out stumbles. None of these parts are readily available, unfortunately, so you're stuck tuning around lean spots with pilot circuit, main jets and needle adjustments only. The point here being that one will have one or the other and the main jet requirements are vastly different. As Jeff has pointed out, an wideband O2 -equipped EGA allows one to quickly figure out what's going on, saving hours of guesswork. [/QUOTE]
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