I have a 1974 Evinrude 135 that has intermittant misfires at all speeds. Sometimes it seems to work normally and a moment later it acts like 1 or 2 cylinders are not firing. Then it will work fine for a while. It does not seem to be dependant on engine temperature. Most of my boating is cruising at about half throttle or waterskiing. Generally there are no extended periods of low speed operation (like trolling). The plugs look wet, but no carbon buildup. There is no apparant deposit buildup on the electrodes or insulator. I am using NGK B7HS10s (not ring fire). Cleaning the plugs does not seem to help, but replacing them helps for a while. It seems that I need to replace them every season. The compression seems good and the coils were replaced this season. Last season, the fuel pump and fuel lines were replaced and both carbs were rebuilt. I am using non alcohol rv type gas. Is it reasonable to replace the plugs each season with this engine? Why can't I clean them? Is there anything else causing this? Could an intermittant power pack cause this problem? Should I try a hotter plug?
