I need help with a popping 07 KX250F

[Torquepipe]

I am about to pull my hair out. I recently bought a pristine 07 Kx250F. I have known the bike from day 1; never ridden hard (older gentleman), only about 10 hours of trail riding on it, bone stock, like new. When I let the bike decel (esp in 1st or 2nd gear), as the decel progresses it will begin to pop & backfire. Other than that, runs great, starts great, idles great.. no other problems, but it bothers me. I started with the stock 40 pilot, backed the fuel screw out in small increments, out to 3 turns out... still did it. Then to a 42 pilot, 1..2..3 turns out.. still did it. Today with a 45, same routine... still did it. The problem seems somewhat better, but still if i let it decel enough without pulling in the clutch or changing gears, it will start popping. My routine isn't exactly like the Eddie routine here, because I cannot access the fuel screw without tilting the carb out, and as I tilt the carb out, the idle changes. But, if I turn the screw all in & tilt carb back to upright the bike will die. However, with the screw backed out to 1 turn or 3 turns, bike will run & idle fine, doesn't seem to make a diff. Hot start cable seems fine (not stuck), air filter is seated fine, can't find any exhaust leaks ( even tightened the header just to be sure). What can I do ? I am seriously frustrated. Thanks to Eddie or anyone else who can help.

[Wayward Son]

a slight pop on decel is normal.

you need to a get a brass extended fuel screw so you set the pilot circuit properly.

[Torquepipe]

a slight pop on decel is normal.

you need to a get a brass extended fuel screw so you set the pilot circuit properly.

Ok, I realize that makes the adjustment easier. However, I have had the fuel screw in every different position in 1/4 turn increments with 40, 42, & 45 pilots. Would you at least guess at why I am getting popping with all the pilots and at every position?

[markvfr]

Hey Torquepipe, I'm going through the same diagnosing nightmare right now and here is my thread. https://www.thumpertalk.com/forum/showthread.php?t=631678 . It might give you some ideas.

Just tonight, I RTV siliconed the header pipe to slipon joint, made sure my exhaust AIS port (you don't have to worry about that on your bike) is not leaking. Will test again tomorrow and see if I still get the backfire. If I do, then for sure its a carb issue.

Good luck. I think we both need it. ?

[Torquepipe]

Hey Torquepipe, I'm going through the same diagnosing nightmare right now and here is my thread. https://www.thumpertalk.com/forum/showthread.php?t=631678 . It might give you some ideas.

Just tonight, I RTV siliconed the header pipe to slipon joint, made sure my exhaust AIS port (you don't have to worry about that on your bike) is not leaking. Will test again tomorrow and see if I still get the backfire. If I do, then for sure its a carb issue.

Good luck. I think we both need it. ?

Thanks, at least I am not alone; misery loves company. I have been working on this thing for a week; haven't even taken it on a ride yet (too embarrassed). I don't know the best way to detect a slight exhaust leak, but I hope that is not my problem. I would really like to solve it at the carb. Thanks again for the help.

[Torquepipe]

a slight pop on decel is normal.

you need to a get a brass extended fuel screw so you set the pilot circuit properly.

Ok... done. I am @ 1 turn out with a 45 pilot. Runs good except popping with any prolonged decel. Do you have any advice ??

[Wayward Son]

its 100% normal.

[Torquepipe]

Ok. Are these KXF's inherintly poppers ? My 06 CRF does not pop at all, even on a long down hill decel. It just bugs me.

[Wayward Son]

your crf is too rich on the pilot circuit if it doesnt pop.

but it can be affected by cam timing,pipe length etc.