Continued Acceleration While off the Throttle

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12/9/2017 7:52 PM

Today while riding my throttle appeared to get stuck for a just a second. I initially attributed it to possible brief whiskey throttle since my forearms were tired, even though I was almost positive I was off the throttle. About 20 minutes later I'm off the side of the track and open the bike up to 4th gear. I let off the throttle and it continued to accelerated. I pulled the clutch in and lock up the back tire and thankfully after about 2 seconds the bike went back to normal. I called it a day after that.

What could cause this? I'm planning on getting into the bike and inspecting this week. It's weird because the throttle does not appear to be sticking and the slide sounds like its closing in the carb when I let off the throttle. Nothing initially seems to be getting hung up with the throttle and feels to be working smoothly.

Any other possible suspects before I start taking things apart? The bike is a 1999 CR250 (2-stroke). I do not race and ride mostly tracks and occasional trails. It does have 49hrs on the top end but the bike has been running like a champ and had not been showing any symptoms of needing it changed yet. Could this possibly be a symptom some how?

Thanks.

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12/10/2017 5:47 AM

Could be that you have an air boot that isn't sealed or has a crack and you are sucking in more air creating a vacuum. Check for any gaps and/or cracks in the intake.

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12/10/2017 11:24 AM

Throttle cable may be getting hung up on something as the slide tries to come back down... I've had this happen on some of my older bikes, right where the throttle cable enters the top of the carb.

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Positively, absolutely 110% obsessed with anything MOTO.

12/11/2017 1:03 AM
Edited Date/Time: 12/11/2017 1:04 AM

I had a similar thing happen to me on a YZ 125. Cost me a painful back injury and 6 months off the bike, the throttle slide stuck wide open even though I returned the throttle tube to a fully closed position. If you won't find any air leaks in the intake, I suggest you tear apart the carb and clean everything thoroughly.

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12/11/2017 1:46 AM

As above, check for air leaks and give the throttle and carb and thorough strip down and clean. Also ensure you are running enough slack in the throttle cable.
Also check your pilot jet, I've found a lean pilot can make the bike hang in the revs a little. Check your air screw and pilot jet size.

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@russ_69

12/11/2017 12:17 PM

Sounds like an air leak. I fought the same issue for a few months. Finally found it, I had bought the bike used and the previous owner installed a V force 3 reed cage and didn't put the gasket on correctly.

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12/11/2017 12:44 PM

I've taken apart this carburetor and cleaned it when I first got this bike. I don't know much about carbs though. Does this little gap inside at the bottom look normal? I checked my air boot and doesn't have any cracks or anything. I've never pulled reeds out before but seems easy enough, and I have a hunch they may be replaced (based on the bike needing some TCL and updating from when I bought it). I'll prob clean the carb / change reeds / new carb gaskets and new reed cage gasket and hope that helps. I may change out the throttle cable for safe measure too.

Let me know if you guys have anymore thoughts!

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