excess rpm & piston seize?

[larrym]

83 merc outboard model without thermostat or temp gauge. Coolant flow normal. My son over reved to 6-6.2k rpm for 10 min. When I noticed, told him to back off to 5.5k. Within 2 min engine overheated & top piston seized. He says over rev didn't burn up engine. What do you think? Also piston melted somewhat, is it repairable?
Thanks for any advise.

 

[JustJason]

Re: excess rpm & piston seize?

repairable... no. replaceable... maybe... but you gotta take it all apart and measure everything out.

Over-reving definitely did burn up the engine.... but you may have had other contributing factors.
The fuel system is not specificly designed to supply the engine with enough fuel to maintain that rpm. What happened is you leaned out, and increased the temprature in the combustion chamber high enough to melt the piston.

When I said that you may have had other contributing factors, I mean that if you had dirty carbs, a worn fuel pump... or other fuel issues, that will only lean out the fuel system further.

Not that it matters now, but the engine shouldn't be capable of hitting 6500rpms if the boat is propped correctly, which it obviously wasn't.

 

[Laddies]

Re: excess rpm & piston seize?

6200 will not hurt your engine, your much better off to be 500 to 600 over the rated RPM that lugging the engine, that amount under the rated RPM. You deliver more water and the engine don't over heat, when lugging a 2 strke it causes preignition and burns pistons not over speed, over speed busts rods. Enough said on that subject, you probably lost the impeller and that caused your problem. You will need to tear it down and probably install a different piston and rings

 

[j_martin]

Re: excess rpm & piston seize?

Like Laddies said, over revving per se didn't hurt the engine. If it over-revved because it was trimmed out too high, it might have been sucking air instead of water into the cooling system. That's what killed it.

Has to be torn down, hole cleaned or renewed, new piston and rings. Maybe damage elsewhere in the engine also.

hope it helps
John