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Honda Cub (Passport) Forums
General
88cc upgrade
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<blockquote data-quote="69ST" data-source="post: 177014" data-attributes="member: 5"><p>Interesting, Roy. I've never attempted this. Of course I've also not encountered a transient lean spot with a stock 16mm Keihin carb, that couldn't be tamed with "non-invasive" tuning methods. It's always been the bigger carbs, VM22/26, feeding 110-140cc tunes...especially the true horsepower tunes, i.e. big valves/big ports or/and carb sized the same as port ID. At one time, Mikuni offered slides with cutaways ranging from .5 to 2.5; now, slide (as well as jet needle & needle jet) availability is spotty, at best. It's frustrating when a 1.5 cutaway is creating an intractable transient lean stumble and a slide change would cure it. With a spare (and otherwise useless) slide on hand, it might be worth a try...maybe even better if one has access to a lathe. I have modded jet needles, with considerable success. That's how I was able to use VM22 carbs on engines with a 22mm ID intake port. That said, it's damned ticklish (read: "high-precision") work. Seriously, being off by as little as 0.0005" can be the difference between best (most linear) throttle response and a steady-state rich misfire at a specific throttle opening. </p><p></p><p>Not raining on your parade here, you may have the answer for a few tuning problems. The skillset + knowledge required is likely going to be beyond a lot of otherwise accomplished shadetree tuner/mechanics. The best low-buck solution is usually sizing the carb 2-4mm smaller than port ID. That's enough to keep airflow velocity high enough to activate the fuel circuits and maintain atomization quality. The best solution, if a 22mm or larger carb can be used, is a Yoshimura carburetor. Their emulsion tube setup is highly efficient...unbeatable, imho...for a price.</p></blockquote><p></p>
[QUOTE="69ST, post: 177014, member: 5"] Interesting, Roy. I've never attempted this. Of course I've also not encountered a transient lean spot with a stock 16mm Keihin carb, that couldn't be tamed with "non-invasive" tuning methods. It's always been the bigger carbs, VM22/26, feeding 110-140cc tunes...especially the true horsepower tunes, i.e. big valves/big ports or/and carb sized the same as port ID. At one time, Mikuni offered slides with cutaways ranging from .5 to 2.5; now, slide (as well as jet needle & needle jet) availability is spotty, at best. It's frustrating when a 1.5 cutaway is creating an intractable transient lean stumble and a slide change would cure it. With a spare (and otherwise useless) slide on hand, it might be worth a try...maybe even better if one has access to a lathe. I have modded jet needles, with considerable success. That's how I was able to use VM22 carbs on engines with a 22mm ID intake port. That said, it's damned ticklish (read: "high-precision") work. Seriously, being off by as little as 0.0005" can be the difference between best (most linear) throttle response and a steady-state rich misfire at a specific throttle opening. Not raining on your parade here, you may have the answer for a few tuning problems. The skillset + knowledge required is likely going to be beyond a lot of otherwise accomplished shadetree tuner/mechanics. The best low-buck solution is usually sizing the carb 2-4mm smaller than port ID. That's enough to keep airflow velocity high enough to activate the fuel circuits and maintain atomization quality. The best solution, if a 22mm or larger carb can be used, is a Yoshimura carburetor. Their emulsion tube setup is highly efficient...unbeatable, imho...for a price. [/QUOTE]
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