78 1150 Mercury fuel question

[Erock]

I bought a boat at the end of the season last year with a 78 1150 motor on it. Haven't had it out yet this year. Went to buy plugs and an impeller today and the guy at the marine shop said I should back the timing down due to the unleaded fuel. He said it would burn up the motor. I never heard of this. Any truth? I don't want to lose any performance, but I don't want to burn up the motor either.

 

[Chris1956]

That guy is mistaken. The correct timing for that motor is max spark advance of 21* BTDC. If it is set to that you will be fine with regular fuel. Some early 70s motors called for 23+*BTDC, which can ping on regular fuel.

 

[444]

To be frank, that mechanic is an idiot. Leaded vs unleaded really only affects the valve seats on 4 stroke motors. Also unleaded had been out for a long time by 1978 so you're fine. While leaded did have a higher octane rating at the time, boat motors such as ours were not built to run right at the ragged edge. Just set your timing as per your factory manual. I've got a 1982 black max 175hp, set as per the factory manual.

 

[Erock]

Thanks guys for the replies. Didn't sound right to me when he said that.