KDX250 Revs randomly

[Dan_1989cr125]

Hey guys! I finally gained the courage to ask the gods here what is wrong with my kdx250! the bike use to run great but would foul plugs at odd times and lose power. I replated and replaced the top end and then the bike would rev really really high and sometimes the kill switch would stop working. the bike lacks compression now and i'm not sure what steps to take next. it is modified with an fmf desert pipe turbine core silencer and carbon reeds. Any steps or tests you feel i should take to resolve the problem? Thanks a lot guys i am in your debt

 

[dirt bike dave]

Not sure about all the other stuff, but revs randomly can be a frayed throttle cable, catchting on the sheath, sticking randomly. You cant really see if it is frayed inside the sheath, so you might as well replace it.

Throttle cables are pretty cheap.

BTW, the stock jetting on the KDX250 is way too rich, which can cause plug fouling and loss of power.

 

[julien_d]

If the revs are running away on you and the throttle isn't sticking, and the compression is low, then you most likely have a largeish air leak somewhere. Check the base gasket, crank case halves, intake boot, and anywhere else you can think of for leaks. With the bike running, you can spray some carb cleaner on these areas and listen for the RPM's to rise. Compression and leak down tests wouldn't hurt while diagnosing an issue like this.....

 

[Dan_1989cr125]

I have had a sneaky suspicion it was an air leak of sorts. Or even bad crank seals. the odd part is how the kill switch fails to stop the bike from revving when this occurs. would improperly seated rings cause this problem? thanks for the help!

 

[julien_d]

it's pretty hard to get the rings improperly seated without it destroying the cylinder in short order. My guess is air leak. And yeah, when the engine runs away like that the kill switch often times will not work. Another cause for this symptom, and one that may be nicer on your ears, is a float level set way too low. If the fuel in the carb bowl is not enough to keep the engine fed, when it starts running on tiny bits of fuel and lots of air it can rev wildly on it's own while making very little power. Has to be rough on the engine.

Now you have 2 things to check ;).

 

[ridejunky]

it's pretty hard to get the rings improperly seated without it destroying the cylinder in short order. My guess is air leak. And yeah, when the engine runs away like that the kill switch often times will not work. Another cause for this symptom, and one that may be nicer on your ears, is a float level set way too low. If the fuel in the carb bowl is not enough to keep the engine fed, when it starts running on tiny bits of fuel and lots of air it can rev wildly on it's own while making very little power. Has to be rough on the engine.

Now you have 2 things to check ;).

Couldn't have put it better....dead kill switch with lean runaway=bad float adjustment or massive air leak.

 

[glad2ride]

After the float check, do a vacuum or pressure test on the engine.

 

[Dan_1989cr125]

Hey guys! thanks! I wil do all of these tests and reply as soon as possible. Thanks for clarifying so much. I will get back to you all soon with the results! :)