Question about Yamaha YZ 250 Motorcycles

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When I start my bike (in nuetral) the rpm goes buck, like its trying to blow itself up, and the kill switch wouldn't work either.. I put octane boost in the fuel..a generous amount, - so I figured that was the problem,so I drained my tank and replaced the fuel with a rich 20:1 premix ratio...and if your gonna tell me that my throttle or carb is buggered please be specific because as far as i can tell my throttle cable isn't to tight and the jet isnt bunged up restricting the needle from sliding in and out...so needless to say im stumped...help me please

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You didn't say what your bike is. All I can do is a generic answer. Put premium gas in the tank. Install a new stock spark plug(s), ( the one(s) you have may be the wrong heat range ). Your bike may have a buildup of carbon on the head and piston. This can create hot spots that can ignite fuel even without the spark plug. The top end would need to be de-carboned. The carbon problem occurs more frequently in two stroke engines.

The most likely cause is air leaks in the intake. Un-carbureted air will make the fuel/air mix too lean. This causes an uncontrollable increase in rpm. Check the metal and rubber fittings from the carb to the engine. Loose connections can let air in. Old, hardened or cracked rubber fittings will no longer seal properly. This includes "O" ring seals. A decompression valve not sealing well would let in air also. Bad head gasket? Loose spark plug? Does your bike have a hot start lever? The related carb fitting may be sucking in air. If I knew what bike you have I could have tailored the answer to your specific bike. Please rate my answer. Thanks.

Posted on Sep 14, 2010

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