Burned Piston, 85 OMC 1.6 L 115hp, 2BBLCO

[Auxlarry]

Had the boat out and after about an hour of wot running she just slowed down and quit, no overheat alarm, nothing! Got it back home and check compression and found the lower starboard cyl at 53 psi. Pulled it down today and found the burned piston and checked the overheat circuit and it is working .
It looks like preignition but I checked timing and was at 24 btdc. Not sure if this is the correct timing for this engine because my book tells me to reference the engine timing tag??
Any help would be appreciated!!

Larry

 

[boobie]

Better check the high speed jet in the carb for that cylinder isn't clogged or partially clogged up.

 

[Auxlarry]

Thanks Boobie!! I will check them since its down and I am waiting for parts. I thought about a carb issue since the other 3 pistons look great!!

 

[boobie]

Let us know what you find.

 

[Auxlarry]

Update:
Pulled the carb apart and can honestly say I didn't find a thing in it!! These carbs got rebuilt last season and they were clean, didn't even find any particulate in the bottom of the bowls. Fuel lines were changed last year as well and I keep two fuel filters on this boat, a water seperator/filter and an inline under the hood.
If the carb didn't cause this, what else could have?

 

[boobie]

This will sound off the wall I know but you could have a power pack that went ape on WOT timing on that cylinder only and caused it. When the job is done, check the WOT timing on all the cylinders. Had it happen to me years ago.

 

[Auxlarry]

It does sound a bit off the wall but I will check it Boobie!! should have the engine running tomorrow so I'll let you know! Thanks for your help!

 

[Auxlarry]

Finally got the engine back together and installed. I found the problem to be the rubber head that goes on the timing stop bolt had dried and fallen off which allowed the timer base to travel further and retard the timing by 8 degrees.....replaced it and reset the timing and the engine purrs on the muffs. If weather allows I'll have it in Tampa Bay today!!

 

[racerone]

???----Retard the timing ?--------Or does that allow timing to advance too far ?

 

[Auxlarry]

When i put the light on it the timing was at 16 before, as I started screwing the stop IN it brought it to 24 before that's when I noticed the rubber cap missing and remember finding it in the pan when I pulled the power head...had to screw the stop screw in 6-1/2 turns to get the timing right. I modifies a part that is similar that we use on our machines at work and got my rubber stop back and re timed to 24 degrees...