I put a SBT short block in a friends 94 650 Montego we broke the engine in as per the instructions. This past weekend he took the boat out and was doing great when it just turn off. He got it to shore and remove the plugs, the rear plugs electrode had completely close (gap) he had a set of new plugs so he change them and continue with no problems for the rest of the day . I told him to drop the ski off so that I can do a compresion test but I was wondering what would cause the gap to close? He did mention that the ground cable was loose becouse he tighten it after changing the plugs as the boat did not crank over.
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Spark plug Gap
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The only thing that causes that is debris hitting the plug. Look at the top of the piston as best you can through the plug hole, and take a compression test. It could have been external debris, or part of the piston, ring, bearings, etc. - No way to tell right now. Since he ran it for the rest of the day, it's possible he got real lucky.
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