Third Time’s A Charm

Q: I have a 1990 Polaris Indy 500 that keeps eating pistons. The first burndown was two seasons ago. The damage was significant, so I did it right with new pistons, rings, gaskets, and a fresh cylinder bore at .020-inch over. After the rebuild the engine ran great and worked fine until I made a long run across a field that resulted in another burndown. The failed piston was damaged on the exhaust side, which I am told is caused by a lean condition. So, I started all over again with another set of new parts. Same story; different day. This time a long run across a lake burned the engine down again. What’s going on?

Paul Rohde

Minneapolis, Minnesota

A: Reading the piston is a very important step to diagnose the problem. In this case, for example, you’re correct to say that damage to the exhaust side of the piston is the result of a lean condition. Lean burndowns are a result of too much air in the air/fuel mixture that feeds an engine. Have you gone up in main jet sizes since boring the cylinders? The larger bore pulls more air into the combustion chamber than before, so more fuel is required to maintain the correct air/fuel ratio. A restricted fuel supply can be caused by corroded main jets. If they are more than a few years old, install a new set. Even a light film of corrosion can restrict fuel flow so it essentially becomes a smaller/leaner jet. Too much air in the mixture can come from several other sources including leaking engine seals or airbox leaks caused by modifications, mis-aligned parts, cracks or unseated carb boots. Engine seals can be checked by starting the engine and spraying starting fluid around the crank bearing seals, head gaskets and base gaskets. If the seal is bad, the engine’s rpm will increase because starting fluid — and air — gets pulled through the seal. A leakdown test kit is another way to test engine seals. They plug the intake and exhaust ports while a mechanic pressurizes the combustion chamber with air. If the engine won’t hold air, a seal or gasket is shot. Leakdown test kits are available through aftermarket suppliers.

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