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Old 09-21-2017, 08:53 AM
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Question 2-stroke sustained rev after throttle chop

I posted this last Oct...

My son races an 02 cr250, mostly stock, fmf pipe, recently added a shorty silencer. 15 hours on rebuild...has always been a little slow to come back to idle but seems to have gotten worse. When riding hard it takes 3 - 5 seconds to come down to idle. I don't know what it could be! Jetting, reeds, air leaks...??

The response I got was to change the left side crank seal. It made a little bit of difference but didn't completely solve the problem. Over the winter it seemed to be less of an issue and into the spring racing.

We did a fresh top end this summer and had the normal slow to come back to idle I expect for the cr250. It was running great and very strong. We have 8 hours now on the top end and the problem is back in a huge way. Almost un-rideable two weeks ago at Muddy Creek MX. When he clutches in the corners I can hear it at a high rev.



My thoughts and questions...

1. If it's the left seal leaking air it would cause a lean mixture right? I pull the plug and it's dark. Wouldn't the leaking lean situation cause a lighter plug? I see a lot of threads pointing to air leaks and leak down tests, but no one tells what the plug looks like.

2. If it's the right side seal wouldn't it leak oil into the case and smoke? This would not cause a sustained rev would it?

3. Reading the plug dark, I moved the needle down thinking I have a rich mixture and it could be loading up on fuel. No change! Should I go farther or smaller main jet? The plug is dark but not dark enough that I think it's a mixture problem...and it runs smooth and no missing.

4. Someone said warm it up and spray starter fluid around the engine to see if I can find an air leak. But again wouldn't it cause a lean plug?

5. Throttle cable is new and very crisp...always has been through the good and bad times.

Frustrated and ready to tare it down if needed but I don't know what to look for or what to replace. I'm looking for some been there done that wisdom. I've noticed a lot of threads state the problem, list suggestions, but never state what the solution was. My name is racer dad, and that's what I am. I'm not a pro engine builder but we do race 3 bikes and I have done about 6 or 7 engines now and this is the only problem I've not been able to solve. Any help would be greatly appreciated!
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