Little Help Please - YZ250F stalls...

[expeditious]

I eed some help from the TT community. I'm working on a buddy's 2007 YZ250F. The bike has a 280 big bore kit. He raced the bike last weekend in a hare scramble. About 3/4 of a lab in the bike stalled on him. The bike was difficult to start. This happened 4 or 5 times during the race. I thought it was a jetting problem initially so i told him to drop it off at my house and i'd fix the jetting.

Well, 5 days later, a JD jetting kit, valve adjustment and numerous jet combinations later, i have the bike running great...when it runs. The bikes starts fine, but again it still stalls. The bike stalls when it drops back down to idle. Even when the idle is adjusted up the bike will just suddenly quit. It's not a violent shut off like it's starving or flooded. It's just a quick shut off like you someone hit the kill switch.

Then i thought it might be electrical. I check the plug (by the way, the plug looked great. Jetting looks spot on). The spark looked good. So now i'm stumped.

Anybody have this issue in the past? Any suggestions or diagnosis would be greatly appreciated.

Thanks.

[mikewrf18]

What jets are you running?

I would guess maybe an issue with your pilot circut.

If your confident it's not jetting or other fuel issue, and your valves are adjusted to spec, have you checked the stator? My yzf went through 2 stators, and when my stator was bad it would shut off when after awhile of running or when it got hot. The first time I had these symptoms it was because my screws holding the stator had backed they're way almost all the way out, and the stator wasn't grounding properly. The second time, the insulation on one of the wires to the stator wore through, and was causing grounding issues. Something to rule out. I'm just a want a be mechanic, so take it for what is worth, but if it was mine, I check this as well. Good Luck.

[expeditious]

main 185, pilot 42, leak 55, 4th grove on blue needle. BTW- bike has 9 oz. fly wheel weight.

[Swede73]

Almost the same situation as this guy.

https://www.thumpertalk.com/forum/showthread.php?t=1024713

[kenpowell]

My YZ290F did the same thing. Took me months to figure it out. Change the main to a 172, pilot jet to a 40 and the leak jet to a 55 if you are at sea level..

[indy rider]

I agree with ^^^, you're probably too rich on the main. I had my ported 269 re-jetted on a dyno after building the motor and it ended up with MJ#160, PJ#42, LJ#50, '09 needle w/the clip in the 4th slot.