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Need help please,I have a 1987 Yamaha 70hp that will run fine and then shut down at high rpms,it then becomes hard to start and idle rough.If I let sit till it cools down it runs fine for a while then it will do it again. I checked ignition with merc spark tester it has fire on all 3 cyl,I pumped the ball made no difference and hit choke and made no difference.
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Need to check compression. Check for spark at the time it shuts off to see if losing spark, if does then test all ignition components. Check to see if fuel primer bulb sucks flat when engine shuts off. -
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I checked compression its within 10psi of each cyl,I have fire on all three cyl with merc testers while its acting up and I pumped ball and made no change. Would high speed timing cause cyl temp to rise and cause a pre ignition and cause it to run rough until it cools off?Comment
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Primer ball is not flat when problem occurs,I also choke the engine while running and it did not make a difference when problem was occurring.Comment
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timing is easy to check with a timing light
and an old motor like that should not be shutting down unless it is seizing up or it is loosing the spark completely.
Testing ignition components like he said is the best thing to do before and while the problem exists to see what is going onComment
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Found problem
The number one cylinder lost compression,so took oil sample from the bottom of the tank and found water.Comment
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