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I have a 2008 YZ450F with a stock header pipe and Pro Circuit T-4 Slip-On Exhaust. I'm 99% sure the slip on is shot because its loud as hell and when I take it off and shake it it rattles. I would repack it but the screws are all stripped ect.

Anyways I've gone over the carb multiple times and I've got it running pretty well. The only issue I have with the bike is that it sputters and pops around 1/2 throttle when I hold it there. When I give it full throttle from any RPM it runs great or when I give it about 3/4 throttle it spins up like a top to redline.

Initially the popping and sputtering was way worse I made the situation better by moving the pilot jet up in a few sizes from a 45 (stock) to a 62. The issue has gotten way better since then but I'm not super sure what to do from here. Should I keep moving my pilot jet up in size because that's a pretty large jump right? I tried changing my needle clip position but that didn't help much if anything it just made starting it worse and little to no impact on the ride.

Any input on this would be helpful. I don't want to buy other slip on. Also as for the rest of the carb I'm very sure everything else in it is what it needs to be at. I've replaced all the gaskets and O-rings cleaned all the jets, valve holes ect. fixed the accelerator pump timing. I just thinks its strange that my pilot jet size is going so far from the stock 45, but all symptoms point to the bike running lean.

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Your needle controls half throttle so put your pilot back to stock and richen your needle.  drop clip,rise the needle

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Fix your air leaks first. A 62 is waaaaaaaaaaaaay too big. All you need is a 45 unless you are at 10k elevation.

Your symptom is NOT jetting.  Jetting requirements do not just change out of no where.

Check you stator harness plug for corrosion, change your plug and check your plug cap for arcing.

Most likely your hot start is leaking, you emulsion tube has oval as is leaking (needle jet), your slide plate seal is worn out, and your header crush washer is worn out. 

Fix all of that before trying to jet away your issues....

Edited by THE KRAN

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