115 Worth Rebuilding?


Last Sunday morning my 1978 mercury 1150, 115hp inline six, failed.  It lost power and before I could shut it off I heard a rapping sound. When I got home I pulled the plugs and number three was packed with aluminum.  I also have the original 1986 115hp powerhead I replaced this motor with.  It has the same problem on cylinder number four. I loved the boat with this setup.  Would you recommend attempting to rebuild this motor?  Are there basic performance modifications I can make like high performance reeds or careful port matching? Lastly the manual I have recommends inspecting but not replacing bearings unless they look shot.  Is this the conventional method or are all bearings replaced?

Some people claim you need to back the timing off to less than the factory recommended 21 degrees and run high octane (92 or better) fuel to keep this sort of failure from happening, do you agree?


Replace all pistons, rings and bearing as you have it torn down, so you might as well do it now. Retarding the timing slightly is good as the fuels of 1978 are much different than we have now. A little safety will go a long way.




Please submit whatever questions you have to IHRTechTalk@yahoo.com.  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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