I have recently found a 1978-1/2 Merc 140hp. Got it cheap. Trouble is, it has a miss in it. I pulled the plugs and examined the pistons. It seems number 6 is black and appears to have started to burn. Otherwise this engine runs smooth (except for the miss). The miss seems to be at all power settings. The engine does not overheat. Someone disassembled a carb thinking a lean condition and re-assembled it. The condition still remains. A friend of mine suspects bad reed blocks. I am a 4 cycle auto engine guy and am still in a learning curve, but I do have a service manual for the engine.

Any ideas?

Compression chck is a must

I had the sane problem with my 1995 115hp that I got used. Dealer told me it was carbs, turns out it was the voltage rectifier/regulator. it shorts out if the battery is hooked up reverse, which the guy I bought it off of had done. good luck with yours.