I have a problem with my mc21. I got fuel in the engine oil, alot!
I also found oil/old fuel in airbox. My gasket in petcock was broken so it not closed when i put it to off. Can this be the problem? I think the floats should close from floading the engine? The bike not run much this days. Maybe one time a month.

It's quite common for there to be fuel/oil residue in the airbox, and if running particularly rich, the airbox can be quite wet. If you ever saw how much fuel "stand-off" is generated by the carbs at high RPM, you'd understand why. A similar effect can be seen here:

Performance Engineering stand-off demo

As for your other problem, it's almost certainly due to a stuck or sticky float needles. I'm terrible for not turning the fuel tap to OFF when I stand my bike up, and I have been lucky to never have had a problem, but really the fuel tap should always be turned off several seconds before shutting the motor off, to lower the fuel level in the float bowls.

If your fuel tap is broken I strongly advise you to replace it, or at least repair it. A few members here have reported being able to drill out the rivets on the tap assembly and replace the O-ring, and then use small screws to reassemble it.

The problem you may face, and may already have encountered, is that neat fuel can possibly damage the crank seals over time.
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Andy.
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Thank you so much Andy! Petcock is fixed, the strange thing is that my carburetors are just newly renovated and the float height adjusted. I will use your tip with turn off fuel tap before shutting of engine. I drain the oil right now, and will have an eye on oil level before each ride.

It may be a silly question, but are you positive it's fuel in the transmission oil? Was the oil very thin, and smelling of petrol/gasoline? Check the 2T oil reservoir... sometimes the seal on the 2T pump shaft can come away from its seat, and this will allow 2T oil to drain into the transmission.

Top tip: Top the 2T remote reservoir off after every ride, and check it before every ride. That way if you see there's no oil in the remote, you know there is a problem with the seal.

Are you confident the float levels are set correctly? They should be set to 13mm [all models].


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Andy.
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I found that keeping the bike on a stand means that the floats aren't on their side, so the floats and needles can close better
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