02 CR250 high rpm bog

[robbieisurfriend]

I just bought a new bike and haven't even got a manual for it yet and have a little problem. I bought it for trail riding but have been riding it out in a big pasture trying to get myself competent enough on it to take it to the trails. The last owner also used it for trail riding so it probably has not been ran like i am running it now. It runs perfect almost all of the time but occasionally when in 4th or 5th gear full throttle at high rpms it will bog out. I am thinking that it is lean because it doesn't feel like "cleaning out" usually feels and it falls on its face till you let off and slow down. Another factor could be that the previous owner ran race gas in it and i am using premium pump gas. He also put in a new air filter. I am just not sure where to start with it; jetting, silencer packing, reeds, RC valve? Is it even possible to get tuned correctly for both putting around on trails AND ripping full throttle in a field? If someone could point me in the right direction that would be a great help until i get a manual. Thanks.

 

[sharky243]

Do you know if the bike has been sitting in storage for any length of time ? Start with the simple things. Do a compression test. It should be around 150 psi. Any less then 125 psi, it may need rings. Check the operation of the power valve (RC valve). Make sure it is free and not gummed up with carbon and sludge. Check the ignition for a good strong spark. Clean the carburetor and take note of the jetting in it. Also check the needle clip possition. Moving the clip down the needle will richen up the mix during throttle opening. While the carb. is off, have a look at the reeds. Make sure they are not cracked or chipped. Make sure they are sealing tight against the cage. Make sure your fuel is fresh and properly mixed. CR250's like 40:1 with a good oil and premium fuel. Make sure your air filter is clean and not over oiled.

 

[_JOE_]

If the engine is tight and sealed, as in good compression and passing leakdown, and the exhaust valve is working correctly you can definately tune the bike to run cleanly at any RPM. You might consider a heat range hotter plug if you know you're going to be doing alot of low throttle trail riding, just be sure to take it out before you go ripping it out in the open.

Premix ratio is a personal preference thing. Just be sure to stick with it after you're done jetting. Some guys run as much as 20:1 for extra insurance and claimed better ring seal and therefore more power. Some people go as little as 50:1 but that would make me nervous.

 

[ellandoh]

sounds exactly like my cousins problem, come to find out ...........plugged exhaust

 

[_JOE_]

That'll do it too.

 

[robbieisurfriend]

Alright guys i will give it a good look over like i should have done right when i unloaded it from my truck but i trust the guy fairly well and he told me he had just cleaned the filter and changed the oil and filled the tank with fresh gas before i left so i had to take it for a spin. Shame on me i know. Thanks.

 

[dirt bike dave]

How does the spark plug look? Is it an old plug?

FWIW, if it was jetted for race fuel it is possible you are running too rich at high rpm on pump gas, and you may need to try a leaner main jet.