I have a 1971 50hp Evinrude Model # E07812, Serial # 50173S. I bought the Clymer manual B734 which covers 1.5 through 125hp engines from 1956 to 1972. While looking up the spark plug information it shows three diffrent spark plugs. The engine cover says to use UL-77V. The book shows that for 1971 to 1972 50hp to use the UL77V just like the engine cover says, but then it says for the 50ES and the 50R to use L77J4. All other engine use the J4J. Right now my engine has L77JCV Gapped at .03. I've never seen a non- adjustable plug before so is the plug i'm running now acceptable and gapped sufficiently or do I need to go buy the plug the the engine cover says to buy. Also can anyone tell me if I have a 50, 50ES or a 50R engine which the manual makes reference to? On the back of the engine cover you can see the word LARK printed on it. I bought this boat and motor as a first boat and every little thing I do makes it get a little better performance. 
I guess Outboards are a little different than lawnmowers.
I guess Outboards are a little different than lawnmowers.