I have a used '02 Yamaha 660 and on last straw with it. Had a motorcycle throttle on it, replaced with thumb throttle. Ran on high idle and couldn't shift it, so adjusted idle and now can shift properly. Now have a sputtering issue that I can't seem to solve. Did easy things like replace spark plug, coil, filters, drained gas, etc. No luck. Replaced carb, spent forever adjusting air/fuel mix, adjusted valves. Helped a bit, but still sputtering. The reverse light flashes every now and again. Asked around a bit with a few mechanic friends. Seems like next step is a valve job or new exhaust. Any advice on here? Seems like a common problem, but not understanding why it won't fix after replacing just about everything.