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General Minitrail Talk
Tech Area
tuning challenge - off throttle issues
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<blockquote data-quote="69ST" data-source="post: 171709" data-attributes="member: 5"><p>The smaller the venturi, the higher the airflow velocity through it. Since carbs work on pressure (vacuum) differentials, higher flow velocity translates into higher efficiency, manifesting as improved throttle response, fuel atomization and fuel consumption numbers. It's moving toward EFI-like operation...up to a point. Just as there is clearly an upper size limit, there is a lower one, also. Go too small and you'll lose horsepower, at best. Beyond a certain point, vacuum levels exceed design parameters and you'll get excessive "pullover effect", where overrich mixtures become difficult-to-impossible to tame...and there'll be a knife-edge separating the effects of tiny changes in tuning/jetting. </p><p></p><p>That said, with a street machine, even losing a little bit of peak power really won't register on the seat-of-the-pants dyno. In fact, average power (a.k.a. output "below the curve") might be better. Thus, you might sacrifice 2-3mph at the absolute top end...but...how much of the time does that matter? Torque is unaffected and that's pulling power + acceleration. If throttle response is EFI-like and the machine pulls itself along with effortless ease, within the range where you spend 99% of your riding time, it can be the better compromise. </p><p></p><p>As for enlarging jets and emulsion tube orifices, unless you have ultra-precision tools/machines and many years experience with this type of modification, the odds of success are low...while the odds of turning parts into scrap metal are high. If you're dedicated/motivated enough to try this, at least source replacements; that way you can bail yourself out, if need be. I've modded jet needles, believe me 0.0005" can make the difference between a slight stumble and a steady-state rich misfire, at part throttle and once you've removed metal, there's no going back. OTOH, getting it right is very satisfying. And there aren't many tuning parts available for carburetors below ~32mm.</p></blockquote><p></p>
[QUOTE="69ST, post: 171709, member: 5"] The smaller the venturi, the higher the airflow velocity through it. Since carbs work on pressure (vacuum) differentials, higher flow velocity translates into higher efficiency, manifesting as improved throttle response, fuel atomization and fuel consumption numbers. It's moving toward EFI-like operation...up to a point. Just as there is clearly an upper size limit, there is a lower one, also. Go too small and you'll lose horsepower, at best. Beyond a certain point, vacuum levels exceed design parameters and you'll get excessive "pullover effect", where overrich mixtures become difficult-to-impossible to tame...and there'll be a knife-edge separating the effects of tiny changes in tuning/jetting. That said, with a street machine, even losing a little bit of peak power really won't register on the seat-of-the-pants dyno. In fact, average power (a.k.a. output "below the curve") might be better. Thus, you might sacrifice 2-3mph at the absolute top end...but...how much of the time does that matter? Torque is unaffected and that's pulling power + acceleration. If throttle response is EFI-like and the machine pulls itself along with effortless ease, within the range where you spend 99% of your riding time, it can be the better compromise. As for enlarging jets and emulsion tube orifices, unless you have ultra-precision tools/machines and many years experience with this type of modification, the odds of success are low...while the odds of turning parts into scrap metal are high. If you're dedicated/motivated enough to try this, at least source replacements; that way you can bail yourself out, if need be. I've modded jet needles, believe me 0.0005" can make the difference between a slight stumble and a steady-state rich misfire, at part throttle and once you've removed metal, there's no going back. OTOH, getting it right is very satisfying. And there aren't many tuning parts available for carburetors below ~32mm. [/QUOTE]
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General Minitrail Talk
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tuning challenge - off throttle issues
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