I recently bought an 450 yfz It ran good for about 10 to 13 hours. Then all the sudden it spits, sputters,and pops through the carb. Took to the dealer they supposedly went through the carb,checked cam timing, crankshaft timing,and valve clearance's. Everything checked out fine they even called YAMAHA and they said to put a stage1 jet kit in so they tried that with with no luck. I don't know too much about carbs but why would it run fine for 10 hours or so and then need jets all the sudden? If anyone could give me a possible diagnosis or explanation I would really appreciate it.