Guest Acemon Posted September 2, 2014 Share Posted September 2, 2014 Took the bike (with newly rebuilt carbs) for a 200 mile ride on Sunday. It stumbles/surges at 5500 rpm, which is about 65 mph at 6th gear. At 4500 rpm it pulls cleanly until getting into the 5000 range, then smooths again starting at 6000 rpm. I called the mechanic (who used to race Interceptors) to explain the problem. He said he cleaned the K&N air filter, cleaned and rebuilt the carbs, drilled out the little plugs to slightly adjust the fuel/air mixture, then balanced everything. He thinks the slow evolution of today's gasoline/alcohol blend causes the mid-range hesitation problem, and to fix it the needles need to be moved slightly (up or down), but the stock needles aren't adjustable. He recommends geting a jet kit (such as Dynojet) which would allow for proper needle adjustment. It means another $150 labor and $125 for the kit, which at the moment I can't afford. This seems a little odd to me. I moved to Denver in 1990 and the bike ran great until I put it into storage (12 years ago), which is when the carbs gummed up. Aside from the K&N filter, the engine and exhaust is complete stock. Could it have been caused by the change to the fuel/air mixture, or is he correct about the needle adjustment? Thoughts? . Link to comment Share on other sites More sharing options...
Member Contributer Dedofnite Posted September 3, 2014 Member Contributer Share Posted September 3, 2014 Ace, The fuel/air mixture screws only adjust the idle circuit. The off idle mixture is balanced by the jets, slide cut out, and needles. He is probably correct about the adjustable needles, assuming the carbs were cleaned and jetted properly. If you keep the K&N a/c, you should ask Santa for a jet kit this Christmas. Ded Link to comment Share on other sites More sharing options...
Member Contributer dalesvfr Posted September 3, 2014 Member Contributer Share Posted September 3, 2014 I'm surprised he didn't test ride the bike before he gave it back to you running rough through the RPM range you would be in - a LOT. Anyway....It could be weak coils too. Have you pulled a plug to check it's condition? Air leaks? Link to comment Share on other sites More sharing options...
Member Contributer IntAceptor Posted September 3, 2014 Member Contributer Share Posted September 3, 2014 On some carbs, the drilled out screws are not just for idle, they influence the mixture up to the mid range and transition to the needle jet and slide. Your problem MAY be a result of the adjustment he made on those screws. It would be worth a try to count the revolutions and adjust them (probably in) to see if that does the trick. Link to comment Share on other sites More sharing options...
Member Contributer Dedofnite Posted September 3, 2014 Member Contributer Share Posted September 3, 2014 IntA, You're right, on some it could influence the small throttle opening transition, like slightly rich or lean at steady throttle causing a stumble when you gas it. I'm assuming the mech made the correct adjustment at idle, which would also be correct at 5000 rpm, but, one thing to check. The K&N could be causing a lean condition there, too. Needles may adjust it out. Jet kit is not required with stock exhaust, but my experience at sea level with the 500s, it didn't cause any weirdness. Not sure about Denver. Ded Link to comment Share on other sites More sharing options...
Guest Posted September 4, 2014 Share Posted September 4, 2014 Ded, The bike is a California model and I bought it when I lived there. Two years later I moved to Denver. The only difference I've noticed is that it runs hot in this thin summer air. You're right about jetting because the Honda service manual says only rejet if it's going to be operated continuously over 6000' elevation. The Dynojet Stage 1 says it's supposed to be for bikes with stock airbox and exhaust, and adds a claimed 5% more hp. For me, I just need the height- adjustable needles. I'm going to do some cautious fiddling with the adjustment screws. I'll let you know the results. If I'm asking Santa for anything, it'll be a KZ-750 Link to comment Share on other sites More sharing options...
Member Contributer YoshiHNS Posted September 4, 2014 Member Contributer Share Posted September 4, 2014 If you want to be extremely adventurous, you could see about putting some small washers (for a #4 or #6 screw) to bump the needle up a bit. Link to comment Share on other sites More sharing options...
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