1996 Mercury 115 2 stroke hole in piston

[vizina]

2011-10-25_15-14-13_575.jpgI just bought a boat that has a 115 Mercury outboard 2 stroke and the guy i bought it from told me it had a hole in one of the pistons, I tore it down and removed the piston, inspecting the other three cylinders and pistons they are in good condition, no scaring on the pison or cylinder walls. The question i have is what is the possible causes for just one piston getting in this shape and not any of the other ones. I've attached a ppicture of the damaged piston, any thoughts on this is great, I'm ordering another pison and rings to replace this one and what to correct any problem that might exist to prevent this from happening again. Alittle info about the motor: The motor is oil injected, The compression on the other three cyilinders is 125-130.

[1Crazymechanic]

potentially clogged oil injection supply?..lean fuel mixture for prolonged period..too "hot" of a spark plug.Which cylinder was the damaged piston in? Post more info..Good luck!

[Texasmark]

Lots of pitting on the crown and the pitting extends into the hole area. Adding to Crazymechanic's comments, I'd say pre ignition damage.....Now, how did that cylinder get pre-ignition? Are the crowns of the other pistons pitted as this one is? I.e. is the whole engine running lean? Where is the spark advance at N and at WOT? I realize that you have it apart and can't make that measurement, but on my 90 of that basic design it is 5 BTDC idle and 22 (timing light) at WOT for a reference measured off #1 sparkplug wire.

That engine is the 4 cylinder 2+2 design isn't it? Not familiar with the fuel flow on that engine (nor the engine for that matter) but it is "different for the lower 2 (idle loafing cylinders). Did he do prolonged trolling?

HTH,

Mark

[vizina]

[QUOTE=Texasmark;3523953]Lots of pitting on the crown and the pitting extends into the hole area. Adding to Crazymechanic's comments, I'd say pre ignition damage.....Now, how did that cylinder get pre-ignition? Are the crowns of the other pistons pitted as this one is? I.e. is the whole engine running lean? Where is the spark advance at N and at WOT? I realize that you have it apart and can't make that measurement, but on my 90 of that basic design it is 5 BTDC idle and 22 (timing light) at WOT for a reference measured off #1 sparkplug wire.

That engine is the 4 cylinder 2+2 design isn't it? Not familiar with the fuel flow on that engine (nor the engine for that matter) but it is "different for the lower 2 (idle loafing cylinders). Did he do prolonged trolling?

HTH,

Mark[/QUOTE
The other three pistons don't have any pitting on the crow or elsewhere. It,s the second piston from the top that this happen on. Once I get it back together I'll check the spark advance to see where it's at, also that carb for that cylinder. Thanks for the info.