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2004 CRF70 Won't Idle Correctly

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I installed a new top end in my son's bike in the middle of last season. New piston, rings, timing chain, valves, and a complete carb rebuild with all new o-rings and jets. All factory parts. With that said I have always had a problem with this bike holding a nice smooth idle. Sometimes it will idle smoothly and then stumble/stall after blipping the throttle and letting it return to an idle. It seems worse when cold and seems like an air leak to me. Valve lash checked outand is spot on. Tonight I noticed that the carb is missing the dust cover for the choke assembly and I'm wondering if this is where I might be getting an air leak. Any help would be greatly appreciated. I am at my wits end and would like to avoid the purchase of a new carb.

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When you can get the bike to idle, spray some contact cleaner on the head/manifold  and manifold/carb areas, and see if there is a decrease/increase in the RPMs and then a return to the previous idle level. That would indicate an air leak in either of those areas. A light coating of a waterproof grease on the o-rings will make a tighter seal.

Edited by dirtbkr188

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Problem solved. I ordered a new idle mixture screw and choke dust cover from Rocky Mountain and installed them last night. I also cleaned the carb again (nothing found). After reassembly the bike runs great and idles like it is supposed to.

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