I've been struggling with the carb of this 2001 XR100 for a month now. I've got the carb off now for the 4th time. When I bought it, it would run but wouldn't idle without the choke on. I know this symptom well and since the bike had obviously not been ridden much and I was getting a good deal it was an easy decision. I tried just cleaning the carb but that did no good. I also noticed when I took out the fuel screw the o-ring and little washer was missing. I cleaned the carb again and fitted a new slow jet (stock 35), carb gasket kit ( that includes the fuel screws washer and o-ring), cleaned out the fuel tank, cleaned petcock and line, new air filter and changed the oil and poured in new gas. Still idles like it's miss firing but runs great above idle. But it returns to idle way too slowly. I adjusted the valves. They weren't off enough to matter. I checked for air leaks in the manifold first with start spray and an unlit butane torch then did a visual inspection. Looked good. Now the strangeness. I just removed the fuel mix screw and the o-ring wasn't on it nor was it in the "chamber". With strong magnifiers and a pin light I could see it stuck between where it suppose to seat and the outlet port. I was able to extract it using a "B" string scrap from my electric guitar with about 1/16" L bend on the end to act as a hook. The o-ring appears undamaged. Every orifice seems to be free of debris. I've been lurking in here for weeks hoping for a break-through before I spoke up. Is there anything else I should check before I reassemble and try again? If it doesn't act right this time I'm going to look for someone in my area with the same bike and see if I can try its carb.