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Yamaha 1998 25 hp 3 cylinder outboard pistons hitting spark plug

I set the gap on my spark plugs, run the boat in the water for 30 minutes, remove plugs and they are slightly closed. i looked at the top of all the pistons and there is a small clean mark where it appears the pistons are hitting the plugs. i can run the motor all day and it runs fine. is there another reason for this other than the wrist pin or rod bearings going bad. it is almost exactly the same on all three plugs

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This must have the wrong plugs in the motor.The gap between the top of the piston and the plug is about 8-10 mm.No motor will run with any bearing that is that worn.You said you looked at the top of the piston.With the head off,did you check how far the spark plug extended through.The reason being,is that it should not protrude all the way into the combustion chamber.It must have the wrong length plugs in it.If it runs well,you have the heat right.Mate.Hope this helps

Posted on Oct 09, 2009

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