250 exc running poorly

[Karna&Justin]

I bought my 99' 250 exc new in 98'. Ever since, the bike has a
occasional (once or twice a year) problem. The bike will be
running fine, but all of a sudden when I engage the clutch, or put it in neutral, the rpm's go way high and the throttle becomes extremely sensitive. The first half of the throttle is super twitchy, and if I open up the throttle the bike just bogs (as if the kill switch was being depressed. The problem continues for 20 seconds or so, and either fixes itself, or the bike dies.

This happens every so often and I'm unable to identify any pattern.
The bike acts as if it's starved, but I can't figure out why.
I've ruled out the follwing causes:

sticking floats (pretty sure)
fuel cap blockage
petcock failure
clogged carb vent tubes

Anybody have any suggestions?
thanks, Justin

 

[RanMan]

Does temperature have an effect on it? Could be lean jetting down low.

 

[Karna&Justin]

Thanks for the suggestion. but I'm pretty sure it's not the jetting. The
"problem" comes and goes a couple of times during a ride. The bike will
be running fine for hours, but then all of a sudden-caplooey! I have'nt been
able to establish any pattern. Also, the "problem" only happens once or
twice a year (except more so the last two weeks).

The reeds are stock (four years old) I visualy inspected them three months ago and they appeared okay, but maybe I shoud replace them just to rule that out also. I have no
idea what's going on!

thanks for your help-justin

 

[MWEISSEN]

I'd venture a guess that it's your float needle. If you can see a wear ring on it, replace it. It could be hanging up, leaning the bike, then finally breaking loose from engine vibration.

 

[fishhead]

does it happen after a long hard pull up a hill or across a mud bog or sand pit

 

[Karna&Justin]

It does seem to happen after getting to the top of a hill, or after
periods of nearly wide-open throttle. As far as the needle being "notched",
I did look specificaly for "notching" and found nothing-
thanks, justin

 

[fishhead]

Might be "run on" caused by a lean condition on the pilot or straight section of your needle or a possible air leak. Does it ping or knock when you let off the throttle on these hard pulls?

 

[Karna&Justin]

Yes, it does seem to ping and knock when the problem happens.-justin

 

[fishhead]

Your jetting package might be a bit lean for high load conditions or you might have an air leak in the intake or ignition side of the crankcase.

The intermitent nature of the problem and the recent increase in frequency is puzzling though.

 

[erider]

check the gas cap vent and also the fuel tap and lines, its sounds like its not flowing enough fuel

 

[MWEISSEN]

How about plugged screens on the petcock assembly (inside the tank)?

 

[fishhead]

you can do a drain test through the petcock with the cap and vent in place to test for blockage.

 

[Scott@w-jinc.com]

I just sold my 99 250 EXC and had had a similar problem. I kept having to richen my jetting, and after running hard it would ping and the idle would fluxuate up and down. The problem turned out to be the crank seal on the magneato side. The harder I ran it the more air it sucked in. Changing the seal solved the problem.

Good luck

 

[lawman]

i had a similar problem on a ported 2000 model 200 exc--after running for a while at hi load, say up a hill or flat out in a field, it would bog as if starved for fuel, then run ok. it also ran as if jetted rich. i nearly went nuts changing parts, finally diagnosed that the float tower in the carb was worn out, allowing too much slack in the float pin, making the float height go haywire, esp. when under a heavy load. i had to buy a whole new carb (owwww!), but it fixed it. on a bike you've been riding since '98, this could be the problem.

 

[yarbonwick]

Also using bad mixture of fuel, low octane rating, or too old of fuel will do that. Which could explain the intermediate nature of it. I know mine is pretty picky in the Summer months. It took me long time to find a happy medium on jetting for my bike, but I essentially did.