What Are the Causes of a Burned Up Piston in an Outboard Motor?

A common cause of overheated pistons relates to insufficient fuel delivery to the cylinder to keep the temperature down. Idle mixture screws that have settings which are too lean will cause a lean air/fuel mixture, and too much air in the combustion chamber causes the temperature to rise. Kinked fuel lines and clogged fuel filters will also starve the cylinders. A rich, or wetter mixture counteracts the condition.

When the cooling passages in an outboard motor become clogged, cooling water does not reach the major components, such as the power head, cylinders and block. Likely candidates for overheating include restricted or clogged freshwater inlet vents on the lower unit, a deformed or deteriorated water pump impeller and contaminated oil which has lost its lubricating qualities and viscosity.

A spark plug that has an improper heat range will allow higher than normal cylinder combustion temperatures and cause localized overheating on the piston top. A too-narrow spark plug gap can also cause early spark fire and allow unburned fuel to turn into carbon deposits.

Dirty oil -- oil which has lost it lubricating qualities and thickness -- will add friction to the rod and crank bearings, piston rings and pins and scoring on the cylinder walls, raising the piston temperature. Piston skirts suffer the most severe friction of scoring and galling, which can melt the piston and rings. Also, improperly mixed 2-cycle outboard oil that does not have the proper amount of lubricating oil will cause rapid overheating.

Pre-ignition results from hot spots on top of the piston when carbon builds up in the combustion chamber. The carbon burns from an afterglow when the carbon has no time to cool down, which causes a premature firing and heats the piston top. An audible "pinging" sound can be heard during idle or low-speed throttle operation. Heavily carbonized exhaust valves allow carbon buildup, as well as excess cylinder pressure. A misaligned head gasket which allows a part of the gasket to overhang into the cylinder can also cause a hot spot.

Detonation results when the combustion chamber contains two opposing hot spots which fire at the same time. It causes a mini-explosion, keeping the piston hot all the time with no chance to cool between strokes. Detonation is heard by a loud spark knock, and the resulting un-timed explosions can burn a hole in the top of the piston.

If the ignition timing is too far advanced, it causes an early spark which fails to burn the complete mixture of vaporized fuel in the combustion chamber. This results in rapid overheating, since the mixture is too lean to keep the combustion temperature down to normal limits.

Old gas which has sat in a fuel tank for four months or more will cause the fuel to thicken with a gum and varnish, which will burn unevenly and poorly. The gum and varnish adheres to the cylinder walls and valve seats, causing an adhesion surface for carbon particles and unburned soot. Low octane fuel also causes uneven and incomplete combustion ignition, clogging the valves and allowing carbon buildup.