My 1975 65hp. merc started fouling the upper plug.<br />It appeared to be aluminum. After tearing the powerhead down, the crown of the top piston was wearing away. The cylinders look perfect. There is no evidence of overheating and it has a new water pump with lots of pressure. There is no scoring of any kind on the pistons. I then installed a powerhead from a "Doner" engine I bought from a previous ignition problem. A few hours on the lake, it started fouling the top cylinder. Coincidence? I was told the top cylinder might have been running lean. I was going to remove the bottom rod and piston from the secong powerhead and swap out the other top piston and rod. This followed by a cylinder hone, new rings, and seals. Any advice or suggestions would be much appreciated. I believe the "Outboard Gods" are punishing me for hooking up my battery backwards at the start of the season
