I was having problems with the engine missing under acceleration, that eventually deteriorated to running on three cylinders. I took it to a Honda shop in town and the mechanic said it needed a valve job. I wasted $1700 on that. It ran the same, but added backfiring to it. I tightened the gaskets and the backfiring stopped. I checked all the electrical components and they were working fine. I took the carburetor off and had it rebuilt. After reinstalling it, the engine will start but runs only on cylinder 1 and 2, whereas before it was only number three that was not firing. There is spark to all the plugs. Where should I look next? The bike is a clean, 1982 Custom 750 with only 17K+ miles on it.