I guess this would be repair. Could be performance.
Anyhow, I've been helping a friend with his Express SR. In the quest for performance, things went south. I got it running and it ran pretty well. We added a somewhat better than stock exhaust - a tecno boss pipe. Ran well with that. We decided to raise the exhaust port. So we did so 1.6mm. Went ahead and replaced the piston and rings and honed the cylinder. I had to go with 2 base gaskets to seal up an air leak so I guess that pushed the cylinder up a tiny bit. Compression is 130 with those two base gaskets.
I heat cycled it 5 times and had trouble keeping it running because previous owner had removed the auto choke system. After I got sick of screwing with keeping it running for the heat cycles, I put the auto choke system back on. It still required too much monkeying around with carb settings to get it to idle right so I popped on another stock carb which seemed to help.
took it out on several break in rides and finally got it to run ok on the last couple. Now it wants to die under load and at full throttle. We've tried 65, 70 and 75 main jets (65 is stock on this bike) without any real changes. Could the crankshaft seals have just taken a complete and sudden dump? Damn thing!(edited)