1/2 to full throttle Problem

History

03 yz250f

New valves, valve springs and new piston

FMF 4.1

Hot Cams

Got bike back from engine builder and installed new FMF pipe and rejetted according to FMF specs

PJ 42

MJ 178

Needle 3rd clip

Starts 2nd kick and idles like a dream. 0-1/4 throttle is strong and crisp. When you go over 1/4 throttle to 1/2 or full throttle the bike seems to cut out like its lean. Pulled the plug after riding at a local track and plug was black and sooty. Didnt make sense. So I tried moving MJ down. I tried 175, 172 and 168. No difference in 1/2 to full throttle?

Eliminated the gas. I run 110 VP. Went to local shop and bought 5 gallons of SUNOCO 110 and did not help...same problem.

Just doesnt make sense. I removed the tank and check all the electrical fittings and they were snug? I'm at the end of my knowledge...which is very little?

Any suggestions or comments welcomed

Run a 45 pilot.

Also install a #40 leak jet. You are forgetting about the AP squirting fuel down low.

Try 4th clip and about a 175-178 main. Just play with those for a while. You may want to try a JD jet kit or 06 needle in the bike.

You played with the fuel screw? Also did you set the float level?

If yuor lean on top as it sounds, you need a BIGGER main and play with the needle like matt said

Mid throttle problems usually relate to improper clip position. Move your clip up one position and try it again. It's difficult to tell rich from lean in regards to the clip, so if going up doens't improve it, try moving it down from its original poistion and try it again. Trial and error really.

Yes I've tried different clip positions. Not much change except from 1/4 - 1/2 throttle which is crisp currently. I will check the AP squirt just to eliminate. Not sure that is problem because its only going "Baaa Daaa Baaa Daaa Baaa Daaa" after 1/2 throttle to full?

Checked the spark plug coil/holder....seemed kind of lose. When I take the rubber boot off of the long spark plug holder and insert a plug....alot of free play. Is this correct or should it fit snug or click in?

Yes I've tried different clip positions. Not much change except from 1/4 - 1/2 throttle which is crisp currently. I will check the AP squirt just to eliminate. Not sure that is problem because its only going "Baaa Daaa Baaa Daaa Baaa Daaa" after 1/2 throttle to full?

Checked the spark plug coil/holder....seemed kind of lose. When I take the rubber boot off of the long spark plug holder and insert a plug....alot of free play. Is this correct or should it fit snug or click in?

That just might be your problem right there, push down on the spark plug cap as far as it will go. There should be no free-play or you'll get a weak spark at WOT and the engine won't fire correctly to clear out the chamber (making it rich and bog).

IF you tried different clip positions, it is now time to try a different needle.

check that plug cap.

Rode it at a little track last night only because the track is small and no real runs to hit the problem area. Guy that owns the track said 1st think that I needed to check my CAMS. He said it sounded like it was a tooth off. So it went back to the engine shop today that rebuilt my engine. Will post back findings.

Oh and the little ignition coil cap for a 03 yz250f is almost $80. Man that hurt but I went ahead and ordered one.

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