Low rpm bog with CR250

[jfletcher]

I've developed a problem with my 02 cr250. In low rmps / maybe 1/4 throttle - 1/2 throttle my bike bogs and misses. Once I get to higher rmp's the bike runs great. This happened suddenly, just during a moto. I thought maybe the slow jet or air screw backed out so I checked the carb but found nothing wrong. Played with the air screw and jet but no luck. This seems an unlikely cause to me since the problem came on suddenly anyway. I checked the reeds and they are fine. Put a new top end in and checked the power valves. They look to be working correctly. So I'm thinking now electrical. Any suggestions?
Thanks

P.S.
This might help
I ride in A class MX, so I'm WOT a lot, and I think some times after it cools down, like between motos, it's ok for maybe 1 lap of the next moto. It's noticeable in timing sections where I need to blip the throttle and and before a whoop section when I nedd to hop in. Also in turns where I need to roll the power on.

 

[Faded]

Things I've noticed in the forums that might cure your problem:

The float level is set too low from the factory. I've read that some guys have had to adjust it 2mm over stock.

The carb bottom is bouncing around making contact with the cases. Wedge a piece of closed cell foam between the carb bowl and the cases.

See if either of these help.

 

[jfletcher]

Thanks, Faded. I'll check those but I'm pretty sure the float level is o.k. since it ran fine all season and then just started acting up in the middle of a race. Everything down there seemed fine when I took it apart.

 

[Faded]

Hmmm,

Have you fixed the air box? I don't want to make anyone feel stupid, but if you didn't fix the leaky air box then dirt could have made it's way into your carb. :think:

Maybe swap out spark plugs. I know more than once I've gotten a bad one.

 

[bedell99]

It sounds like you have an air leak somewhere. Double check your rad valve gasket, Reeds, even case. I had the screw come loose the holds the carb clamp onto the reed valve and caused the same symptons. Thats where unwanted air can find it into the engine on this bike. Also as faded said it might be something as a sparkplug or missed connection.

Erik

 

[jfletcher]

Yep, I changed the pulg with the new top end and I fixed the airbox when I first got the bike a year ago. The reeds are sealed good, after the top end job I checked that well.
It could be an air leak in the cases somewhere though, I'm not to sure how to check that. Like I said, it happened all of a sudden in the middle of a moto so something obviously "broke".
Would an air leak cause this only at low rmps?

 

[BEEF706]

Have you checked the electrical connections to the RC valve? It is acting like the valve isn't working right, and since it came on suddely it could be that. Let us know what you find, this one is a puzzler.

 

[UKcr125rider]

Originally posted by Faded

The carb bottom is bouncing around making contact with the cases.

The cr125 has a little piece of rubber stuck to the case directly below the float bowl bung, and absorbs any impact if the carb moves. Try sticking a little peice of soft rubber on.

 

[jfletcher]

I'll check thoses connections when I get a chance. My instinct tells me it's electrical, although if you listen to the bike it sounds like a jetting type problem but riding it I can feel it's "missing" down low, like it's stuttering before the RPM's get higher.
There is a little piece of the crankcase gasket broken and sticking out on the bottom of the cases. As if it was tightened to tight and broke the gasket. And a small amount of oil leaked out over the winter, but it's never been apart and I don't know if that would cause the problem (air getting in there seems unlikely to me, but what do I know I can't get this figured out anyway)

 

[slickpuss]

Sounds like what I went through with my 02 Cr. After about 6 months of riding, the jetting started changing. Nothing helped it. So after trying everything to fix it, i put a new top end in. Then I found oil in the crankcase. The seal has been leaking and making it run to rich. My bike would bog, smoke out the track and then take off. This might be the problem if you have a lot of smoke.

 

[Faded]

Originally posted by slickpuss
The seal has been leaking and making it run to rich.

What seal?

 

[slickpuss]

Sorry, the primary side crank seal.

 

[jfletcher]

Good suggestion Slick, I didn't mention it, but there is no excess smoke at all. I have an 03 cr250 that I think I might start swapping electrical componets with to see if the problem goes away. I'm not sure where to start though. It's too cold in Michigan to ride outside so it's kind of a pain to diagnose the problem since it runs fine on the stand.