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01 YZ250 miss only around 1/4 throttle

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Been working on this bike for the past week. When riding for a good 1/2 hour before working on where the problem is. Fixed most of the issues with the top end.

2001 Yamaha YZ250

Stock jetting is 178 main and 50 pilot

stock needle is N3EJ middle position

I am running 50:1 92 octane pump gas and power plus oil

Air bleed screw is 1.5 turns out and idle screw is 2.5 turns out

Right now it's jetted 185 main and 52 pilot

N3CW needle 3rd position.

Using a BR8ES plug.


FMF fatty pipe w/ shorty silencer

Twin air air filter

Things I have done:

I disassembled the carburetor and cleared all passages and am using new factory jets.

Checked for vacuum leaks around reed cage and carb boot.

I bought this bike used and don't know about the needle that is in there now. I plan on trail riding and hill climbs. I ride it around in 1st gear and slowly roll on the power. Everything after 1/2 throttle is good. Instant wheelie when @ WOT. When I ride around holding 1/4 throttle, it misses alot. I shut the bike off and checked the plug and appears to be lean.I live in HI so elevation is @ sea level. Temperature doesn't seem to matter. Just wondering if it is normal to run this rich of a jet. Will do a compression check tomorrow @ work.

Very :banghead:

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if it is lean,it should run better with the air screw closed further.

if it is better with the air screw fully closed but still not right go to a bigger pilto jet and retest.

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That is what I was thinking. Pretty frustrating, but I think I'm getting the hang of it. BTW, I found out something interesting. It runs better with the choke on. So I guess I gotta go richer. Thanks again. Very good forum.

Thumpertalk rules.

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