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Message: [QUOTE="landover50, post: 1478651, member: 146252"] All, First post on this board, hoping I can get some help. Bear with me, it's going to be a long post to explain everything. I bought a 2009 YZ450F with a Keihin FCR carb from a neighbor who had ridden about 10 hrs in 8 yrs. It only ran on choke when I bought it. I pulled the carb, cleaned and rebuilt it (pilot jet was completely plugged). I replaced all the other jets as well but left the needle and clip. I also replaced the spark plug for good measure. It ran better but was still having low throttle issues, sputtering/hesitation and backfiring on decel as well as the lean bog. I adjusted the fuel screw the best I could but slowly it got worse and worse, having major lean issues (pipe got red hot after only 2 minutes of idle in the garage). So I pulled the carb again to take a look. When I pulled the fuel screw (I installed an after market/adjustable screw) only half of the oring came out (yes it was installed in the correct order....screw/spring/washer/oring). The other half was lodged in the passage between the fuel screw and the pilot jet. I blasted it with some compressed air and it came out. Thinking I had found the issue, I put it all back together. Of course it ran better than before with the oring lodged in there, but it was still sputtering/hesitating at low throttle and still had lean bog issues. I ride at 5000-8000ft so figured I may need to lean it out a little so went down in size of pilot from a 45 to a 42 and I also did the oring mod and installed a smaller leak jet to help with the lean bog. Still had low throttle issues. So I pulled the carb again and rebuilt the mid body (gaskets seemed a little flat but didn't seem to be any major issues there) and completely cleaned it again and had it ultrasonic cleaned as well. While I had it apart this time I also checked valve clearance and they were spot on. I put it all back together and still low throttle issues. I adjusted the fuel screw to super lean, maybe 1/4 turn out, and it seemed to get a little better. So I installed a 40 pilot and put the fuel screw 1.5 turns out. Still issues. So again I put the fuel screw maybe 1/4 turn out. It helped the sputtering but it is still there and I have a lot of backfiring on decel. It's rideable but from what I have been able to read a 40 pilot jet with a really lean fuel screw doesn't seem right. Now I'm at wits' end , not sure what else to look at and do. I haven't messed with the main jet or needle/clip position since I don't seem to have issues in the mid to full throttle range. Am I missing something? I'm not a professional tec but this is the second Keihin carb I've owned and I've read extensively on it. Hoping I can get some tips/advice from someone more knowledgeable than me. Thanks in advance. [/QUOTE]

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