wierd idling problems

I recently purchased my 99 YZ400, and i really enjoyed riding it one time. MY complaint with the bike is a wierd idle. If the bike is idling on a crate it is fine. If you attempt to lift the bike off the crate and lean the bike the idle jumps about 500 RPMs. The same if I'm riding the bike, and it also occurs when you blip the throttle. It will remain at a higher idle speed for ten seconds then come down. I have checked the TPS sensor and it seems fine. The carb is cleaned thoroughly and all cables are possitively routed correctly. The bike is completely stock besides a 180 main from a 175 stock and the needle jet clip is moved from the fourth to the fifth. I also feel comfident that there is no air leak from the intake manifold. If any of you guys have had a similar problem and know the remedy i would greatly appreciate it. Thanks thumpertalk

I have a 01 426 that started doing the same thing after I had it about 3 months, it would remain at high idle for about 10 seconds before returning back to normal, but seem to be worse after it was at running temp.

I talked to thumper racing and he asked if I jumped the bike alot and I replied yes. They said to check the float level because the tang between the 2 floats is probably sprung.

Sure enough I tweaked the tang up a little and cured the problem.

Let me know if this helps, It helped me !!!!!!!


Beaumont, Texas

thanks jay bird! I took apart the carb earlier today to clean it again for the fourth time this week. I just checked my float level. According to the brief sentence in the manual. I think it is within specs. The manual says to turn the carb upside down then rotate in till the needle jet aligns with the float arm. Float Arm? What i did is turned it onto its side so that the float tab is touching and closing the needle valve with out pushing in the spring loaded part on the back of the needle valve. When i did that my measurement was exactly 9mm which is spec.


-Did you check to make sure the hot start is closing tightly? and the hose for it?

-Did you look at the float needle and seat closely for blemishes that might make it stick? Jason's symptoms sound like yours too.

Tipping the bike to get a fast idle is odd. :)


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