WR250F Throttle run on

**K6K** · 18th October 2009, 19:04

Today on my third lap of the WMCC two man race my Yamaha WR250F started to run on when off throttle (like the idle screw was set very high). With the clutch in it would drop back to low idle revs after a few seconds but it was running pretty rough and stalling easily. It did not seem like the throttle cables were sticking but judging by the revs it almost seemed to be sticking on about 1/8 throttle (but only for 3 or 4 seconds or so). When coasting downhills it was popping a hell of alot.
Running on the needle or main jets was fine so I just had to run pretty hot into corners...
I'm going to strip and clean the carb and try and figure out what the problem is tomorrow - I've changed all of the jets before but never really stripped a carb before.
Any ideas on the culprit or suggestions about what I should look out for?

**humai** · 18th October 2009, 19:22

Sounds like it's taking additional air into the inlet manifold. Did you check the clamps on either end of the carb, particularly the engine side, to see if they're still tight? How about the hot start button (if fitted) - is that still snugged home or has it moved for some reason? Check the whole inlet system, including the various WR-only emissions plumbing add-ons that are usually blanked off by the dealer, for possible air leaks.

Good luck.

**Rupe** · 18th October 2009, 19:53

Originally Posted by K6K

Today on my third lap of the WMCC two man race my Yamaha WR250F started to run on when off throttle (like the idle screw was set very high). With the clutch in it would drop back to low idle revs after a few seconds but it was running pretty rough and stalling easily. It did not seem like the throttle cables were sticking but judging by the revs it almost seemed to be sticking on about 1/8 throttle (but only for 3 or 4 seconds or so). When coasting downhills it was popping a hell of alot.
Running on the needle or main jets was fine so I just had to run pretty hot into corners...
I'm going to strip and clean the carb and try and figure out what the problem is tomorrow - I've changed all of the jets before but never really stripped a carb before.
Any ideas on the culprit or suggestions about what I should look out for?

Not sure on the set up of your bike, but does it have a hot start lever? Could it be stuck open? The little piston bit that goes into the carb can get pretty gummed up.

That excessive popping is ment to be a lean idle jet, but will also do it if the hot start is sticking.

other than that, yeah strip the carb, and throttle assembly, and clean.

**tnarg** · 19th October 2009, 11:55

I've had my hot start stuck before, kinda sound like what your talking about. Might be worth checking that out.

**K6K** · 19th October 2009, 17:08

Verdict: The rubber on the airbox side of the carb was off slightly, once it was put right I could see how the carb was slightly off angle. Also the hotstart plunger plastic doesn't seem to screw in very well as the plastic threads arent great though I dont think that was the problem. I'll replace it anyway. The bike started straight away and ran as normal after reseating the carb. Hope the engine didn't eat to much crap with that air bypassing the filter....