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Mikuni Carb On A Zero


flint45
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I have a Mikuni carb on my 92 Zero that I assume is maybe a VM 26? The BETA parts breakdown shows a PHBH 26 which is maybe a Dell Orto and the Mikuni was put on years ago? Any way the bike runs fine until you get past 1/4 to 3/8 throttle then it is way rich and smokes like a trash fire. Plan to clean the out side then undo the top and lower the needle if it's not already to see it that cleans out the high speed circuit.Any other suggestions? Many thanks.

Edited by flint45
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The problem may not be the carb. Rather the crankcase is full of oil from years of putting around.

Opening the throttle increases RPM which in turn increases vacuum frequency sucking up oil and crud from the crank case.

May just need a good run out at high speed to suck and burn it all out. If the carb was too rich it would not smoke but just bug down and die.

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Things I had done beforehand where new clean air filter, cleaned entire exhaust. I ran a couple passes in fourth gear that should have cleaned it out but maybe not. Going to at least check to see if the main jet is loose. It ran this way when I got it so who knows. I'll check the other things mentioned. Thanks

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Thanks Dan. I just got through running it and made several high speed runs and it's past the smoking like a trash fire and pulls a little longer before it flattens out. Should it keep pulling like a 2 stroke enduro or is that just the manners of the trials engine? I will check that flywheel as lineaway mentioned also and mix a batch of 80:1 next time I buy fuel for it. It's my first trials bike so still learning their mannerism. First night with my new Gaerne balance pro boots also which allow me to ride much longer without feeling like the pegs are cutting my feet off.

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Yeah after a few years with Diadora boots I'm back to Gaerne and happy as they are soooo comfy. I think Billy had it right with build up of oil in the crankcase. It's not uncommon for even a new bike to smoke out on the trail after plonking about in the woods and at 50:1 I would imagine it was pretty bad. If I remember the '92 flattened out pretty early. You can't ride 'em like enduro bikes. You get the best by short shifting and staying in the meat of the torque curve.

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