bike doesn't want to stop?

i recently sold my cr250 for a 99 yz400. I'm very pleased with the bike so far, but i think there may be a problem with the bike. First of all it has a very sporatic idle. My second problem is when i approach a berm and cut the throttle and pull in the clutch the motor seems to rev at 2000-/+ RPMs. The throttle cables are functioning fine and are routed correctly. I just cleaned the carb (stock pilot jet and 180 main. I believe the problem is from an air leak or a crank seal that is leaking. If anybody has a simular problem or knows the answer to my problems, it would be greatly appreciated. Thanks thumpertalk

Does it just Idle high when you are coming into a corner or anytime you let off the throttle. How long does it idle that high before it goes down to normal.

We'll it's not your crank seal, and my 426 manual doesn't spec a main jet any larger than a 179. I run a 165. You may want to double check those numbers before you do anything else.

Other that that, 2000 rpms is a lot. I don't think you would get that much gain from an air leak.

So you're either getting more fuel from somewhere, or your timing is radically changing.

Remove the cable cover from the carb and the next time the idle goes up, make sure the carb is really closing.

If this is fine, check the TPS sensor (see the manual).

Hope this helps.


302, don’t forget this is a Thumper :) It doesn’t have crank seals, the crankcase is actually vented and adjoins the gearbox. The whole thing is oil-fed, including the flywheel/magneto. Ring-dings use the crankcase as part of the induction tract so they need crank seals.

Did this problem surface after you cleaned the carb?

If not then I’ll assume you got everything installed correctly, but it still sounds like it has to be bad carburetion or a leaky manifold. Although I wouldn’t completely discount the timing/TPS issue that DaveJ mentioned.

I guess the only thing I can offer is double check both intake clamps in front of the carb, put the idle mix at two turns out, reset your idle speed (w/ bike warmed up) and see what that does.

Other than that if you didn’t clear out the pilot air jet and starter jet when you cleaned it I would check these as well (there’s more stuff that can go wrong with the FCR than most two-stroke carbs).

thanks guys. looking through my manual I just figured out what a tps sensor is. I have not been around four stroke dirtbikes too long. It seems like a pretty good possibility that it may either be my tps or mixture problem. Thanks again for the knowledge.

i did the test of the tps sensor as directed by the owners manual. There seems to be no problem and it passed all the tests. My next idea is the valve clearances, but i really am running out of ideas on my problem. if any body can help?

It sounds like a lean situation in the slow circuit. Try turning the fuel screw out 1/4 of a turn, test to see if it goes away. Continue at 1/4 turn intervals until it improves or you reach a max of 2 1/2 turns out from fully closed. After that try going to a 48 pilot jet and begin again. What position is your needle clip in? Stock is #4. Your main jet seems fairly rich at 180 (175 is stock). Do you have an after market exhaust system?

DaveJ the 400's have different main jet sizes than the 426's. When I bought my used 99' 400 it had a 182 in it, for what reason I do not know. The plug was black as all get out.

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