This week my buddy and I put a new piston in my '09 YZ250F. Took the cylinder to our local Yamaha place and they did a gorgeous hone on it, too. So, new piston and rings, fresh hone, valves checked (no need to adjust - gotta love Yamaha!) So we get it together last night, and it's go time: we kick it about 30-40 times and it finally starts. I put this down to fresh metal internals needing to get used to each other. Finally starts, and died. Start it up, it's clearly unhappy, doesn't want to stay running, so I give it some gentle gas to keep it running. Eventually it calms down, but when I roll off the gas, it dies. Obviously to do the piston, we had to take most of the bike apart. We had the carb lying down for about 2 days, and I wonder if maybe something got clogged in there? Also, the clamp that goes from the carb to the intake tract of the head cannot be re-used; the bolt that tightens it got stripped out before we even took it off. So we sliced off the rubber slit that keeps it from rotating, and put on a hose clamp that fit. Up top in the RPM range, it runs great. A lot more power, a given since the blow-by on my old piston was nothing short of spectacular. Two problems: when I roll off the gas, it hangs a good 5-7 seconds before dropping back down to idle (running a 42 pilot, stock - sadly we didn't check and clean the carb while it was out of the bike. sigh.) when it drops down to idle, it will start to run rough, and then die. Spark plug looked fine, I have a small radiator leak, but my coolant level is good, so I don't think it's running hot. Obviously we are working on the radiator issue. I haven't put a second heat cycle on it since I took it home last night, and was planning on it tonight. Thoughts, anyone?