bike starts and runs on 3 cylinders. Occasionally the other will fire, but won't stay firing. I've replaced the CDI, plugs, wires, and plug cap ends. The plug from the bad cylinder is soot covered. I checked the feed from the coil and it is sending the current to the plug. Carbs were rebuilt last year, but it was running the same when I got it back. Please help! I don't know where to look next.