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Mikuni Carb Jetting (Methanol)


dag8501
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I recently purchased a bike with 38mm Mikuni's bored out to 41mm. It run but not very well. Slight bog off the line and breaks up a little on the top end. Seem very rich blowing raw methanol out the pipes. Can anyone please give me some advice on the jetting/needle for these carbs for the following conditions:

Sea level, 200 psi compression, plus 8 timing, 7mil, drag ported cub. I think it currently has 55 pilot and mains drilled out to .116, not sure on the needle

I have trouble clearing it out at the line so the needle may be too rich. Also, I was running it with the power jets screwed in 1.5 turns from wide open.

Thanks for the advice!!

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  • 2 weeks later...

Ohm test your plug caps and coil. Every bike likes a lil something different but you’re dump size seems small to me not rich. When you’re at the line trying to clear out keep it under the rpm where it sucks through the power jet. Lean your needle to take care of launch and then set your power jet doing plug check on plug bottom using burn pattern. Don’t get to hasty n do one thing at a time. I’m not gonna tell you start changing dump jets and a bunch of other shit if you don’t get back with real time data on what each move has done. I’ve squeezed down power jets to completely closed on a few bikes. 1/2 a turn at a time until you get a 180 burn . None of it means anything if your electronics are not up to par 

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Thanks for the feedback. I checked my ignition and everything seems to be within spec. It is also worth mentioning that my bike in equipped with a fuel pump and billet bowls. At K&T advice, I did change my fuel valve to a 1.8 but could the pump still be causing a rich condition? Should I set my floats a little leaner, perhaps 20mm?

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  • 2 weeks later...

Did you verify with k&t the needle and seat combo. I’m not well versed on mikuni carbs aside from smaller gassers. If that’s not rite it will push through . 7 mil cub by me at sea level 39keihin bored out. Tuned in at .1225dumps,.121 mains ,60 pilot Usually an E series needle. There’s also a lot of little things like is your power jet drilled out to far, angle of power jet, height into the carb bore . if your electronic check out start squeezing her down and read the plug , what’s your plug gap set at. , what plugs are you using. What were the ohm readings on your electronic components. 

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  • 2 weeks later...

I did talk to Kim at K&T and I am running the correct size needle valve, 1.8 if I remember correctly.  I also did some further investigating and while my needle jet is drilled to .116, my mains are actually .102, which seems very small. I dropped that needle all the way down to the leanest position and installed a 40 pilot, but it still revs like it is too rich. And that is with the leanest needle made for my carbs. Maybe I need to drill out a new needle jet a little smaller? At wide open throttle it runs well with the power jets only 1.5 turns off of seat.  Not sure why this motor does not need much fuel. My plugs are gapped at .18. I did not ohm out my stator but the coil primary was .4, the secondary was 6200 and the caps were about 5000. 

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  • 1 month later...

Finally got a chance to test the bike after removing the pump and it ran worse. The right side is still rich, but now the left cylinder plug is showing a lean condition. Got home and vacuum test the motor and it did not reveal any leaks. The carbs have large bowls, but could it be running out of fuel on that side? What other reasons would cause a lean condition on one side if the motor passes a vacuum test? Do mikuni's only run well on alcohol when using a pump? Or should I throw the mikuni's in the garbage and get me a set of PWK's? Because I am about to that point.....

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