250F Idling problems

[Mike112]

Hey guys, I have a 2010 kx250f I am having trouble with keeping it running. Any ideas on what it possibly could be?


*** A little history on the bike. I got the bike from a friend. He was out riding one day and the bike would continuously shut off unless you were in the throttle. He let it sit for a year and a half never touched it. He was told by a guy at the track the bike may need a valve adjustment.. and he never touched it from there. When I got the bike from him I was looking into it and notice the fuel mixture screw was missing. I got one and thought that may be my problem.. well here I am now..***


-I can only get the bike to run with the choke on and when I turn the idle screw down it idles perfect but as soon as I turn the idle screw back up to normal sounding with the choke on, the bike will idle for a few seconds and die out. Also I did notice the squirt from the accelerator pump is really weak. I have tried adjusting the screw to find the “sweet spot” still can seem to find it. I’ve tried the o-ring mod as well on the needle and still can’t figure it out.


While taking the cylinder head off I noticed a hair line score. Not any noticeable compression loss though. (I plan to rebuild that once I can figure this other issue out)


(I do not know when the previous owners rebuilt the bike!! I know it has been rebuilt or messed with because there was gasket maker between the valve cover gasket and the head.)


-I have noticed the cam chain seems to be stretched out and loose a little bit but I don’t think with it being stretched out would cause this problem. I may be wrong on that I’m not exactly sure, correct me if I’m wrong please.


-I’ve already rebuilt and clean the carb several times, upped the pilot jet (42 and 45 from 40) and went down in the leak jet as well (45 and 50), (55 was already in it when I got the bike)


-I’ve did a valve adjustment on it and triple checked everything and one of the exhaust valves is out of spec already.


Any help would and is GREATLY appreciated, Thanks!


-Mike

 

[SRAD97750]

Hi.
Your symptoms scream pilot jet being clogged. I know you cleaned/replaced it, I'm just saying the symptoms still point to idle circuit being at fault. The choke bypasses the idle circuit, which would let it run choke only.
Also, valves being out of spec can cause alot of rough running issues.
-BIG DAN

 

[Mike112]

Hello,
I know it’s funny because I’ve tried cleaning it out several times most of the times I take it out to check it or swap it, it seems to be fine. What or where could I start to diagnose this problem? Valves and cam chain and get those in spec to 100% be sure that’s not the problem? Also how would I be able to tell if the valves and springs are worn and need to be replaced?

 

[SRAD97750]

I would get the valves in spec at least, then you can try tuning the carburetor more finely.
Once valves can't be shimmed into spec any longer, they are needing replacement. -BIG DAN

 

[Mike112]

Thanks for the reply!! Ok so it doesn’t matter if the shim size? As long as the valves can still be shimmed they are fine, is what you’re saying? (Just want to make sure I understand completely before I go ahead and look any further into this). Should the valve spec fall in between the clearance spec or should it be more towards the higher or lower end of the clearance?

 

[Mike112]

Okay I will see what I can do and I will get back to you. Thanks for your help it might be a little I just had a hip surgery 3 weeks ago so I’m recovering from that right now! Thanks for your help!

 

[SRAD97750]

I like it on the thick (high) side of the clearance, but can make for a noisier valvetrain. Gives the valves longer to wear.
Valve adjustment is a slippery slope. Usually a motor will stay in spec for a long time, then once you have to adjust it, they need adjusting regularly until they are toast.
-BIG DAN

 

[timoyz]

And just because the pilot jet is clear does not mean the circuit throughout the carburetor is also. Then to the valves, once you start adjusting no more than four times before you should do a complete valve job.

 

[Mike112]

Okay I will try my best and seee what I can do and I will get back with you guys in a few. Might be a week or so for the parts to come in. Also once I adjust the valves more than 4 times you think it’s time to go ahead and replace them? I’m also going to get a new cam chain as well and clean the carb once again.

 

[Mihylo33]

Okay I will try my best and seee what I can do and I will get back with you guys in a few. Might be a week or so for the parts to come in. Also once I adjust the valves more than 4 times you think it’s time to go ahead and replace them? I’m also going to get a new cam chain as well and clean the carb once again.

Solid advice from Srad and Timo. Always replace that cam chain when you swap out the valves.