99 YZ400f Backfiring and Lingering Idle

My 400f backfires when you let off the throttle from a rev and it will hang high for a second or two before dropping down to idle. Now i have been told that this might me an air leak but im not sure. I also have a hard time starting the bike. I kicked it today for about 15 min straight and no start just backfired two times. Im not sure if its out of timing or not. I also need to check to see what size jets I have in it. Any help or suggestions would be great.


If the valves are in spec I'd agree with an air leak or that it is jetted really lean. I had air leak problems in 03 and since then I have kept an eye on things related to intake and the intake bell's o-ring.


Thanks for the reply and the advice. I am going to tear into the carb this week and see whats going on. I checked the timing earlier today and it is correct. I think that the carb is clogged up with something. Thanks for all the help.

You could be right, but don't overlook your valve clearance.

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