Mercury Inline 6's Fuel Grade??

[JimbC]

What is the best fuel grade to run in these older motors. Regular, Mid, Premium. And WHY?
??

 

[mr 88]

Unless your timing is retarded a few degrees there is a good chance you will fry it -burn a hole in the top of the piston with todays crap gas.The extra dollar or so that it cost to fill up a 6 gallon tank with premium is well worth it vs the expense of a rebuild or another engine. I believe Merc years ago even put out a service bulletin to its dealers tellling them to back off the timing. I run premium along with adding a gallon of 110 race gas every time I fill up a 6 gallon tank. My timing is set at pre crap gas numbers. Depending on what your mainly using the boat for may determine what grade gas to buy or where to set the timing. If you back it off you can run maybe mid grade but you have to ask yourself , Do I feel lucky.

 

[wired247]

The original timing setting on the 1500 was 23 degrees. It runs best like that if you have good gas. In the mid 70's it became real hard to get gas that was worth a flip and Mercury lowered the recommended timing spec to 21 degrees. Todays 92-93 octane pump gas will allow for 23 degree timing. Anything under that you should stick with 21. If its a 90 HP or 1150 and are using the original low domes don't sweat it and run 21 with low octane crap gas. Anything over 130 psi cranking compression go with higher octane gas. Anything under 130 don't worry about it.