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 September 3, 201411 yr by flint45

The `92 came with a Mikuni. Should have about a 140 main and 27.5 pilot. If your carb is set right, I would pull the flywheel and check the keyway as they broke often.

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.

Probably has an exhaust system that is fairly gunked up too.

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

1/4 to 3/4 should be the needle/needle jet. Maybe yours is worn. I know it happens to older bikes. What are you running for oil/fuel ratio?

Dan I mixed it at 50:1 with Belray synthetic and 102 octane. Thanks for the replies guys.

50:1 is pretty rich for a water cooled trials bike. I typically run no more than 80:1

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.

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.

The `92 would rev out good for a trials bike, but maybe not as much as you are looking for. If the bike has cleared out the excess fuel, then the key way should not be an issue.

Thanks guys. I dont really care to run it that hard. Maybe it's okay and I will run it and find out.

Appreciate the help. I rode it for 45 or minutes last night it it hardly smokes at any speed now and seems to run pretty well. Ill start a new topic with the next issue which is the front brake.