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Overheating jetski. Any ideas

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I have a 1995 Kawasaki 750 STS jetski. The problem was when I bought it, that it ran for about 5 minutes, then would lose power, and eventually would not restart at all. After pulling the engine apart, I found 1 burned out piston. We put 2 new pistons in. The problem was still the same. We then found that the relief valve was broken (you could blow through it both ways). It seemed that maybe there was a vapor lock being created. We replaced that valve, but still the same problem. It seems that the magneto box is overheating. Not sure if this jetski is supposed to shut down when it overheats as a safety precaution or not. The main question is, what needs to be done to stop the overheating.
 

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Overheating jetski. Any ideas

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I have a 1995 Kawasaki 750 STS jetski. The problem was when I bought it, that it ran for about 5 minutes, then would lose power, and eventually would not restart at all. After pulling the engine apart, I found 1 burned out piston. We put 2 new pistons in. The problem was still the same. We then found that the relief valve was broken (you could blow through it both ways). It seemed that maybe there was a vapor lock being created. We replaced that valve, but still the same problem. It seems that the magneto box is overheating. Not sure if this jetski is supposed to shut down when it overheats as a safety precaution or not. The main question is, what needs to be done to stop the overheating.
The blown pistons make me think you have an air leak in the crankcase making the engine run lean. When the engine runs lean it will have early detonation (spark knock) which over time will blow holes in the pistons. If the engine is overheating it may be the head gasket, especially if the stream is weak out of the bypass. The overheating can cause the crankcase seals to deteriorate and start leaking (see above)
 
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