Why does my bike do this?

[Broken Spoke]

I have a 98 CR250. I just rebuilt the bottom and top ends. It has been running great, cranks easy, snappy response, but it idles funny. After it gets warm, it revs up and down when idling. It doesn't rev high, it just kind of surges. I thought about it being an air leak, but everything else seems fine. What do you think?

 

[trial_07]

In what shape are the reeds? Did the bike have this problem before you rebuilt the engine? When you say rebuilt bottom end, what did you do exactly? More details please.

 

[Broken Spoke]

It has new reeds. I do not know if it had this problem before I rebuilt the bike because it would not idle at all and I could not make it idle. As far as rebuilding the bottom end, I split the cases, put all new bearings in the transmission, new crankshaft and rod, new crank bearings and seals. Thanks for the reply.

 

[ktal74]

I had the same issue with my 97 CR250 before and after the rebuild. That surging was insane. I too totally rebuilt the engine last spring. New piston, new crank, new V-Force 3 reeds, etc... I believe it was detonation when the motor got warm.

What solved this issue for me was running higher octane fuel. I now run a 50/50 mix of 110 and 92 with Klotz R-50. NO MORE SURGE or detonation. It now runs very smooth.

There may be other ideas, but for a few extra bucks I believe this will solve your problem.

Good Luck!

 

[darringer]

If it surges at idle when you come to a stop after riding it usually means a lean pilot circuit. Try adjusting the air screw by turning it in 1/4 turn at a time. If it stops surging count how many turns out the final setting ends up. If it's less than 1 turn out, the pilot jet is too small and should be replaced with the next size larger.

 

[CRazy250]

how funny does it idle? is it hilarious?............yeah sounds like what everybody is describing, usually my bikes do that too when they are out of fuel and sucking in air.