Vintage Dirt Bike Q & A

Recent full engine rebuild including rebore and Wiseco piston kit.

engine starts fine then loads up on fuel.
Gas spits back out carb into airbox.
Float level is set at 20mm as per spec.
All jetting is stock.
Cycle was standing 22 years and excellent condition, apparantly running ok when stored.

I have fitted a new carb kit , float valve etc.....no improvement
Turning fuel tap off will eventually clear it out.
I lowered float level progressively with only minor improvement

I blocked off choke circuit .....no improvement.

I fitted a late model RM125 carb to see what happens and engine runs much cleaner.

Any suggestions please....

Carb is priginal 36mm Keihin and looks in good order!!

THanks,

Sounds like you have already replaced the needle and seat, but if you only replaced the needle, you might have a bad seal under the seat which would allow fuel in even when the float is shut off. Most Keihin carbs don't have a replaceable seat but if memory served me correctly they do use an o-ring under the seat to seal some of them. I've also seen Keihin carbs where the seat is pressed in and there is no practical way to remove it and replacement seats are not available. If you have not already tried this take the carb off and check the float seat by blowing in the fuel inlet and seeing if it seals when the float is shut. Sounds like you have replaced the float itself, but if not you might have a fuel saturated float. Fiber floats can look just fine, but sink like a rock due to saturation. Is there any fuel coming out of the overflow when you run the bike?

Yip, found the problem..the float sinks!
Have tried soaking it in various substances without success.

Local Honda dealer cannot source a new part.he has no part # listed in parts book..

Can anyone suggest a suplier for a new float.
Thanks
John

Aside what has been mentioned, have you looked hard at your intake/exhaust manifold. If the valves aren't closing fully or if timing is off it will cause gas to be pushed back into the intake manifold and out the airfilter. Or worse, past the rings into the crankcase.

Bob

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75 CR 250M1 Elsinore
83 Rokon Trailbreaker
99 BMW R1100RT

You might try http://www.sudco.com who import Keihin and Mikuni carbs. They have a PDF catalog for download and when I have called them they have been very helpful.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF