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Detonation

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As many of you all know I now own a Skater with 98 300ProMax. I always check the plugs and pistions after a day out on the water. Last time out I noticed #1 piston dry and not oily especially near the top. All other pistons are oily over about 90% of the dome and black in color. Now I have read that the PM coils fail a lot and wondering if this is what's happening. Also noticed at idle, only at times I can here the engine miss or make a pop sound once or twice. Also have heard the same thing around 3500-4000rpm and the engine tone changes for a 1/2 second or so and repeats several times. What are the signs, and what do the pistons look like when on the beginning of detonation. I have already cleaned the injector screens and bench tested the injector spray patterns. I did move injector #1 to #6 before going out today but piston still looks somewhat dry. Piston dome also has a slight brown color along with black. Anyone got pictures of the pistons or know of a sign which does.

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Your problem sounds like #1 is leaner than the others. If the other cylinders are black, they sound a bit rich. A nice brown color is good, but tan is getting lean for a general purpose lake engine running pump gas. Excessive black soot/carbon in the piston indicates an incomplete burn in the chamber.

If your ProMax has gaskets, the first sign of detonation will be small dents or a rough sanded feeling on the inside of the metal sealing ring on the gasket - usually on the exhaust side. The EGT's will drop slightly on the bank that has detonation, and power will drop slightly.

If this gets bad enough, it will cause small pock marks to be blasted away from the top of the piston and even the head - about the size of the tip of a pencil, and power will drop off. The EGT's will drop considerably, and the engine temp will go up. From here the heat will get bad enough to start weakening the surface of the piston, and small areas will start to melt. If the ring land above the ring stays intact, the piston will over-expand and score the bore. Eventually you will catch a ring, or melt the top of the piston, and lock up the engine.

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When I purchased the engine I had to replace no. 3 piston and cylinder sleeve. I have attached the pictures after removing the head then before the rebuild. First picture is for #3 cylinder and the other is #5 cylinder which is about what my #1 piston now looks like except it's very clean near the top 50% of the dome (up & S) markings. Comments

I ordered 6 new coils because of what other enine builders have said.

post-1-1123118594.jpg

post-1-1123118707.jpg

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