I have an '90 xr 600. It ran and started great when I first bought it about a year and a half ago. One day it just died when it was at idle. No sputter at all, like there was no spark. Soon this happened a couple times per ride. Then it started to have fits of REALLY hard starting. It was completely random, fall or no fall, hot or cold, anything. I also have a '96 xr 600 which runs perfectly, and I tried swapping a few parts in an attempt to locate the problem. It (the'90) has a strong spark, the spark timing's good, valve clearance is good, and I've tried switching the carb and plug with the ones from the '96. A less minor problem, the '96 seems to have much stronger snap off the bottom. It has a 155 main, 65 pilot, stock air filter and a big gun silencer. The '90 has a K&N filter, a supertrapp, a 160 main and a 65 pilot. could it be just the pipe that makes a difference? Any good ideas? It's driving me nuts, any help would be greatly appreciated.