Idle problems/Gearing questions

I've got a completely stock 99 WR400, including the throttle limiter, and I have a problem with the idle surging. Starts, runs fine, but when the engine is hot sometimes the revs jump up for no apparent reason. The revs sound about the same as when it's cold on full choke. Blipping the throttle doesn't do anything - the only thing that seems to work is to put it in gear, let the clutch out to put a load on the engine till the revs come down. Then you can put it back in neutral, or pull in the clutch and it's fine, at least for a while. It happens when I'm stopped and when I'm running it. It's a real pain when you're coming up to a tight corner in the trees, roll off to slow, but the bike just wants to keep going. Any suggestions??

Second is a gearing question. I'm stalling this thing a bit too often and having trouble going up rocky trails - can't seem to crawl smoothly. It's got the stock sprockets and I was considering going to a 52 tooth rear. Is this a good start? I don't want to think about a flywheel weight, at least not now.

Sounds like you may have an air leak. I use carb cleaner (some us WD-40) and spray it around the intake boot and carb to see if the motor picks up RPM. If so, you may have a leak. You may also want to consider inspecting your throttle cables for freying.

For the stalling, yes a 52T will help. Use the stock chain and it'll move the rear wheel up to make it easier to turn in the tight stuff. Some have even gone to a 13T countershaft sprocket.

Oh and cut that throttle stop and really enjoy the power you have :)


Bill, not sure what your response to YZ timing was, but when I just did my '01 it seemed to actually improve what stalling I was experiencing. Certainly contrary to what I expected. Before the timing change it would shut off like a light when lugged too low. After the YZ timing I was amazed that I didn't stall once that day I rode it and I know it would have stalled in a couple of weird places I got into.

And yes, gear it down. Either a 13t front or 52 rear...real improvement!

Thanks for the tips, guys. I will check for an air leak, but I would think with a leak, it would run rough all around instead of just having the revs run up intermittently. Never hurts to check, right? As for the rear sprocket, I'll definitely change that this winter when I do the swingarm and axle service. What's the deal with the YZ timing? IS it that big a deal?

I have the same problem when the fuel screw is not properly adjusted. If I turn the screw *in* 1/4-1/2 turn from the on my stock 426, the idle will "hang" like that (I do adjust a little depending on how cold/hot/humid it is outside, etc).

Try adjusting your fuel screw out 1/4 turn at a time to see if that helps. Most recommendations are not to go over 2 turns. If you need more than 2 turns to improve things, you need a different pilot jet.

DO you have any popping on deceleration? If so, combined w/your idle problem, that would indicate a lean condition in your pilot circuit...

Steve T

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