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Melted piston on a KZ R2
yeah jetting is the first thing I'd look at on a KZ when a piston goes. mine just melted and I dont know if that's it or if the water wasn't moving. like a tractor that overheats when the belt slips. anybody know what usually does that
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The piston crown fails when combustion temperature exceeds what the aluminum can take, and on a KZ that almost always means the mixture went lean under load. That is the usual cause. A cooling circuit that has stopped circulating will overheat the engine, but that tends to seize it rather than locally melt the crown the way a lean main jet does.
The water pump on these engines is gear driven, so the belt-slip comparison does not apply. If flow actually stopped, the fault is usually an airlock, a damaged impeller, or a blocked radiator. Confirm water is moving before you fit another piston, or you can repeat the failure for a different reason. If the jetting was not obviously lean and it still melted, check that ignition timing is not over-advanced, because that will overheat the crown with a mixture that would otherwise be safe. When you are unsure on the carb, richer is the direction that protects the engine.
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I'd get on that radiator first thing on a dirt oval. That thing packs up with clay in a hurry and you cook a piston even when the pump is doin its job. Pull the screen and see if the fins are plugged, ain't gonna hurt nothin to rinse it while you're in there.
If the timing is runnin a hair too far advanced it will put a hole in one just as quick, that is something folks argue about in the pits plenty.
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Coolant sitting still will melt a piston pretty near as quick as anything else. these water cooled mills dump a lot of heat through the crown into the head and if the water aint circulating that heat has nowhere to go.
A fella can see the pump turning and still have the impeller shot, a thermostat hung shut will do the same thing. I would make sure the water is actually moving before chasing the carb. A rich mix won't save it.
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I'd skip tearing into the carb until I know the water is actually circulating. You can lose a whole Saturday putting a piston in and still melt the next one if the pump isn't moving anything.
Feel both hoses after a couple minutes on the stand they should both get warm pretty quick. Packed radiator fins or air left in the system after a refill will cook a piston just as fast and it costs nothing to check. Kid sitting there waiting, that's the first thing I'd look at so you're not still in the paddock when practice starts.
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