Weird spark missfire

[oldnbold]

My 03 250f has a weird, repeatable, miss at a specific throttle position. Specifically when you 1st crack the throttle from idle and hold that position, in any gear, it has a constant missfire. Open the throttle up and it acclerates perfectly w/0 miss or hesitation. Close it and decels w/o backfire.

Things I've checked and done:

1)turned fuel/air screw from 1/2-4 out w/o noticeable effect. Same results.

2)Squirted carb cleaner through the idle air circuit looking to see if I had no-toil rim grease sucked into the circuit. (I thought I was onto something with this as it had some in the carb boot. No effect. This procedure was a campside attempt and my screwdrivers would not unscrew floatbowl so I had to settle for the clearner through the airjets on the intake bell. Used 1/2 can with spray tip.

3)changed the spark plug. No effect

4)checked the carb boot for air leaks. It is fine.

Any ideas? The only thing I can think of is maybe the TPS has some grit or something in it to send a false reading to CDI. This problem happens at a specific throttle position.

I have not checked valves in 30 hrs (hourmeter) but it has no symptoms of bad starting or anything. Hot or cold same starting w/0 issues.

I did find it odd that my tampering with the fuel/air didn't seem to affect much at all.

[barton]

I had a very similar problem on my 03 when I first got it, and it drove me nuts. Finally went up on the pilot jet from 40 to 42, and that cured it. Give it a try and see what you think.

Obviously you'll want to re-adjust your fuel screw with the bigger pilot jet.

[B86_Racer]

Chris-

Did the problem just now begin to occur? Mine will do the same thing but believe it or not, ever since I moved to the #40 leak jet, my bike runs alot more crisp off idle.

[oldnbold]

Chris-

Did the problem just now begin to occur? Mine will do the same thing but believe it or not, ever since I moved to the #40 leak jet, my bike runs alot more crisp off idle.

No, this prob started being noticeable about 10-15 hrs ago but on the mx track that throttle setting doesn't last long enouggh to bug me. this weekend went woods riding and was at this setting far too often and it was a PITA to hear it missing so much.

42 pilot or 40 leak jet, huh. I think I'll go the leak jet route as occasionally I get a bog when I whack the throttle. I thought everyone was nuts about this problem until it happened to me. ?

Thanks Donovan.

[B86_Racer]

Good choice, Chris. You're welcome and I hope it works out for you.

[barton]

FYI, the leak jet only has effect when you're twisting the throttle. It is used to meter the amount of fuel that the accelerator pump squirts when you whack the throttle. While I don't doubt that B86's bike runs better, and doesn't bog with the smaller leak jet installed, I don't think it will help the problem you describe, since it appears to be a steady throttle issue. The leak jet may fix your bog though.

Just my $.02

[oldnbold]

FYI, the leak jet only has effect when you're twisting the throttle. It is used to meter the amount of fuel that the accelerator pump squirts when you whack the throttle. While I don't doubt that B86's bike runs better, and doesn't bog with the smaller leak jet installed, I don't think it will help the problem you describe, since it appears to be a steady throttle issue. The leak jet may fix your bog though.

Just my $.02

Barton, I think I'll take your $.02 also as I'm looking at a whopping $8-10 repair bill for the both.

[barton]

Yeah, big bucks, huh? ?

[acguy]

Looks like you may need to run a smaller leak jet and of course check timing of your AP squirt. Go to Thumperfaq, and refer to AP mods. A lot of the dialog referes to 2001-2002 bikes, but you can definitely learn from it.