1985 75HP Merc eating top piston

[Kawigamog]

At short burst of full to high speed aluminum chips caused spark plug to fail - once removed engine would run fine until short burst at high speed again - top cylinder only - new piston last fall - visable damage on raised part of piston - afraid to run engine at high speed again - why only top piston? if solid state ignition problem why only top cylinder? problem with crankshaft -spun- no visable damage to lower unit any suggestions

 

[rodbolt]

Re: 1985 75HP Merc eating top piston

its either lean or hot. some of the blocks had steam bubble issues so we located the tell tale to the top of the block, check your ign timing at WOT, not at cranking. check your fuel delivery system,not just carbs.<br /> something is creating heat in the combustion chamber that is leading to an uncontroled burn of the fuel mix. also known as detonation. as you can see it can actually eat pistons.<br />probably ate the first one as well.

 

[JB]

Re: 1985 75HP Merc eating top piston

Merc Trouble

 

[Kawigamog]

Re: 1985 75HP Merc eating top piston

Originally posted by rodbolt:<br /> its either lean or hot. some of the blocks had steam bubble issues so we located the tell tale to the top of the block, check your ign timing at WOT, not at cranking. check your fuel delivery system,not just carbs.<br /> something is creating heat in the combustion chamber that is leading to an uncontroled burn of the fuel mix. also known as detonation. as you can see it can actually eat pistons.<br />probably ate the first one as well.

 

[Kawigamog]

Re: 1985 75HP Merc eating top piston

steam bubble issues in casting? why should this cause a problem? going to make sure water pressure is high enough to cool top cylinder - make sure prop matches the boat - what does WOT stand for? - using fresh gas and oil - retarding ignition - changing carb jets to richen mixture - any other suggestions?

 

[Texasmark]

Re: 1985 75HP Merc eating top piston

WOT is balls to the wall.....wide open throttle. Grin<br /><br />If you have a thermostat, take it out and look on the pellet for a temp stamping....like 120....stands for 120 degr F where it is supposed to open.<br /><br />You can put it in a pot of water under low heat on the stove with a candy thermometer stuck in the water and check it out.<br /><br />May not buy you anything but you never know. I had an OMC once that acted real strange and later found out that the thermo was stuck closed....course that cost me a powerhead as I was running at WOT when it (the engine froze up and then I found out that it had a (stuck closed) thermostat.<br /><br />Mark