96 Evinrude 112 special, burned piston

Bob Gale · #1 December 5th, 2008, 03:18 PM

My 96 Evinrude 112 special has a melted/burned #2 piston. The engine runs, but has no compression in #2. It was overhauled at the beginning of the summer and had run fine during the summer. What can cause a burned piston? Timing? Running to lean? Help

Dhadley · #2 December 5th, 2008, 03:22 PM

Several things can be the root cause. Describe what you mean by a burned piston. Is there a hole in the dome? The corner by the exhaust ports burned off? Dome OK but sides are scuffed?

When it was rebuild in the summer was it because of an issue with #2?

tashasdaddy · #3 December 5th, 2008, 03:23 PM

were you in the navy in Jacksonville, early 1970's? timing, lean, predetination. have you pulled the head, to see the condition. does not sound like a head gasket with 0 compression.

Bob Gale · #4 December 6th, 2008, 12:07 PM

Thanks both for you replys,
Regarding the discription of the burned piston, it looks like the top has been melted evenly across with a torch, burning about a 1/16 on an inch off and depositing alum on the cyl walls. The other cyls are ok. The rebuild was for a different problem. Someone had mentioned an ignition pac double firing, never heard of it??
PS no not in the Navy, in the CG in the 60's but not near Florida.

Dhadley · #5 December 6th, 2008, 12:17 PM

If it's burned the sharp edges of the dome i's an ignition related issue. What spark plugs are you using? What was the reason for the other rebuild?

emdsapmgr · #6 December 7th, 2008, 05:22 PM

Did you pull the carb apart and look at the high speed jet on the side of the carb where the piston failed?

Bob Gale · #7 December 7th, 2008, 05:54 PM

Hi, thanks to all for the good ideas, hoping to look at some thing suggested Monday. The HS jet sounds like a winner also, running to lean will do that? What happened to the good old days when the HS jet was a big black knob in the front of the cowling?

Bob Gale · #8 December 9th, 2008, 08:03 AM

Hi again, The original rebuild at the begining of the summer was for a broken piston in #4. The top of the piston started coming apart and the top ring broke up. Hmmm, sounds like it may have started as a burned piston?? The plugs are QL77JC4 Champ. We had looked at the carbs during the rebuild and didn't see anything wrong. Guess it's either running lean or timing? Anyone have any info on timing?

tashasdaddy · #9 December 9th, 2008, 08:10 AM

Bob, if you don't have an OEM manual, get one. http://www.outboardbooks.com/index.c...996&ID=8616 7

Dhadley · #10 December 9th, 2008, 09:05 AM

Pistons don't normally break, especially on top. Most likely the edge of the piston was burned away or broken off by shock waves - probably at the exhaust port. Again you are looking at some sort of ignition event.