2007 YZ 250f won't accelerate off bottom

My bike is nasty off the bottom and I can't seem to eliminate the problem with jetting and I was wondering if anyone could help? I have verified engine timing and valve adjustments, new spark plug, new stock accelerator pump diaphram, clean K&N air filter, P/C pipe, R&D Bowerbowl with adjustable leakjet. Accelerator pump squirt verified not hitting bottom of slide. Carb is very clean, no clogged passages or jets.

Main- 180

Needle- OBELQ 4th position

Pilot fuel- 45

Pilot air 105

Fuel screw 1 3/4 out

Leak jet set 1.5 out to #40, adjusted from #10-#70

Below are combinations I have tried in last week or so

Mains- 170 & 178

Pilots- 42 & 48

Needles- OBEPQ and JD Blue

If I crack the throttle down low in 2nd gear bike studders like hitting a rev limiter and then takes off, will pick front end up within 30 feet or so on flat ground. Not a lean pop where it will die, not a rich condition where it will accelerate slowly, it studders like a rev limiter almost. When in 3rd gear steady 3/8 or so throttle hit it and it stands right up and accelerates cleanly, which leads me to believe main jet circuit is properly jetted. Bike idles at 1950 rpm, starts easily with choke cold, needs hot start when hot which leads me to believe pilot jet circuit is jetted properly. I bought bike used, appeared to have been raced, had typical lean pop when I test rode it.

Thanks in advance

Sounds like what was wrong with one of my buddies bike and it turned out to be an electrical problem.

Does the bike have the stock AP cam spring (Often, the R&D items com with a stiffer one of fewer turns). If you are unsure, do the oring mod, it will not hurt and may be part of the solution. Next, with the bike fully heated, tweak the AP timing screw, turn it in 1/2 turn at a time and then test by riding (not just reving in the garage). Keep careful track of how much you tweak the screw.

Zanetrain13- could you tell me what the electrical issue was?


William1 I installed the stiffer springs when i put the bowl on. I have tweaked and turned the acc pump timing screw so much I should probably replace it due to wear. Thanks anyway

If you bike runs fine at a steady state (ie, you hold the throttle and it runs perfect) your problem is the AP, though checking the TPS may not be a bad idea too.

i believe it was the stator

turn the ap timing screw fully in.better?


Turning AC pump timing screw all the way in with bike fully warmed up and riding it has little to no effect on problem. Turning it all the way in when carb is off bike eliminates any fuel from coming from nozzle when throttle wheel is turned, which I believe means it is operating correctly?


lets back up.

has the bike ever ran correctly?

suddenly started doing this or appeared after "mods"?

When I test rode the bike at the dealership it had an off idle stumble which I thought was accelerator pump circuit. Fundamentally the problem has been the same since day one.

it was a new bike?


In my first post I mentioned that it was used and appeared to have been raced. I probably should mention it had a powerwing in the intake tract and a powershot bowl on it also. Lastnight just to verify I turned acc pump timing screw in all the way after it was warmed up and it reduced the problem by maybe 15%, not much of a change but a change non the less.


you put the powerbowl on it? same with power wing?

powerbowl installed w/ adj leakjet

powerwing fell apart, might be in garbage


Did I answer your question fully?


its hard to say at this point. so many variables.

the issue only occurs when you open the throttle fulyl and quickly from low rpm?

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