115 I4 Merc engine piston failure

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nofaultxx39

Cadet
Joined
Oct 1, 2008
Messages
7
My 99 Merc 115hp I4 blew a piston. The #2 from the top is the piston that went. The whole top looks like it has been melted and sandblasted or something. Half the piston skirt also broke off. I got a used block and will reuse the old carbs, etc. With only 1 cylinder having the issue what could have caused the failure. I can't attach the pics as the resolution is too high on them.
Thanks.
 
Faztbullet

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,620
Re: 115 I4 Merc engine piston failure

From what you are describing that is detonation. This can be caused by restricted high speed jet(lean),wrong heat range spark plugs or defective trigger. Rebuild carbs and clean, replace impeller, new correct plugs and replace trigger.
 
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nofaultxx39

Cadet
Joined
Oct 1, 2008
Messages
7
Re: 115 I4 Merc engine piston failure

Thanks for the suggestions. However, could you elaborate on the possible trigger malfunction? Would it be firing prematurely, or at will, or what? If it didn't fire or fired late I would assume detonation would not have occurred.
 
Faztbullet

Faztbullet

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Re: 115 I4 Merc engine piston failure

I have seen them cause a switchbox to "double fire"
 
C

CharlieB

Vice Admiral
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Apr 10, 2007
Messages
5,617
Re: 115 I4 Merc engine piston failure

The Force black boxes would also go bad and "Double-fire", leaning out the Dbl'd cyl and toasting it. I regularly tested every one by using an ignition test light while running, look at each cyl's timing mark and again 180 out from it's mark. If the mark appeared twice it was time for a new box.

Maybe I should start checking again.
 
Faztbullet

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,620
Re: 115 I4 Merc engine piston failure

Any time I get a blown engine that I cannot lay my finger on what caused it, the ignition system gets a major look at. I have seen failed bia's circuits that lets packs run away,scr's fail and 1 cylinder fire all the time. In over 35 years you see a bunch of weird,unexplainable crap.
 
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