Got myself an old Yamaha XS 500B towards the end of last year and although the bike runs, its a bit erratic and doesn't idle well, when I started it today I noticed petrol coming out the overflow on the R/H side, anyway I decided to remove carbs with a view to giving them a good clean thinking they were maybe gummed up but it turned out they were actually pretty clean, however I discovered the vacuum piston on the right hand side was stuck in the up position (left hand piston was in correct low position), the piston has some scratches on it as has the inside of the chamber which seems to be the cause of it sticking, and the spring is not able to push it back down easily due to this, cleaned it up anyway and got it moving a bit easier but it still has a tendency to stick occasionally due to the scratching! is there anything that could be done? or will I need to try and source replacement parts, the carburettor is the Keihin type as used on the earlier models and don't know how easy it would be to get spares?
Yamaha XS 500B 1976
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As you know the needle valve/s is/are not shutting off the petrol,if the float/s do float then check the float heights (upsidedown 22mm i think) and try to blow through the fuel inlet tapping the carb (vibes) for valve seating.Scratches (no air filter?) polish the upper bore with solvol autosol or toothpaste.Also fit an inline filter between the fuel tap and carb.I`m fresh now.
Thanks for the reply, will try the autosol/toothpaste thing, there is an air filter however the plastic airbox was out of shape a bit at outlet and no hose clips had been fitted so was not doing its job, this has allowed dirty air into carbs causing scratches, have previously fitted in line fuel filters.