Only one plug fouled

[huntin dawg]

Gents,
On my 1973 7.5 HP Merc only one of the two plugs shows any fouling. It has only been run in a tank since the lakes are all frozen. It idles like a dream and revs great. I pulled the plugs today so I could treat it with Sea Foam and just one plug showed signs of firing. The top plug was clean and the bottom plug black. It couldn't possibly be running on just one cylinder could it? Seems it would run like crap or not at all on just one jug.

What would cause just one plug to show signs of firing?

I plan to check compression on it.

BTW, the cylinder with the clean plug has scoring on the piston that is visible from the exhaust port. However, the rings are still free and compression feels the same by plugging the spark plug hole with my finger and pulling it over.

Any ideas would be appreciated.

HD

 

[achris]

Re: Only one plug fouled

If you want to make sure each cylinder is firing and contributing evenly to the engine, using a pair of INSULATED pliers, pull one of the plug leads off. The engine should drop and you should be able to hear the spark in the plug boot. Push it back on and pull the other. It should drop about the same amount.

A very clean plug could indicate water getting into that cylinder.

Chris.......

 

[huntin dawg]

Re: Only one plug fouled

The last time I test ran it I had replaced the exhaust cover gaskets and they were leaking like crazy. Can water enter the cylinder from the exhaust side? That's the only way I know water was introduced to the engine outside the cooling system.

HD

 

[achris]

Re: Only one plug fouled

The last time I test ran it I had replaced the exhaust cover gaskets and they were leaking like crazy. Can water enter the cylinder from the exhaust side? That's the only way I know water was introduced to the engine outside the cooling system.

HD

HD,

Water can (and does) get in from the exhaust side, it can also get in through the lower crankshaft seal. If the powerhead was removed to do the exhaust gaskets, then the lower seal should be done at the same time.

C.......