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TOPIC: '92 75 HP Mercury 3 cylinder runs half speed

'92 75 HP Mercury 3 cylinder runs half speed 1 month 1 week ago #137277

  • dspeck
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I bought this engine two years ago for my '61 G3 Centerdeck. It ran well on the motor stand and well initially on the G3 last summer with a top end of about 44 mph. This year I swapped props (from 14.25 X 17 to a 15 X 21) to see if I could get a bit more top end. I accomplished that - maybe 2 mph more - and it really jumped when I hit the throttle. Then, two weeks ago, I ran out of gas. I purchased fresh 89 octane gas and took her out for a run. She started and idled well but was slow to accelerate when I took it to half throttle. It came on plane but top end was just under 25 mph. There was no discernable miss-firing or stumble. Thinking I might have sucked up some sludge when I ran out of gas, I took her apart to clean the carbs. In so doing, I discovered that the synchronizing bar to the top cylinder was broken off just above the top screw. That explains the lack of power - the top carb wasn't opening. Replaced the broken part and went ahead and cleaned the carbs just in case. Put everything back together and resynched the carbs. She started and idled beautifully. However, when I got her back in the water, the problem is still there. No guts and top end half of what it should be. Am now wondering about a bad coil pack. Any suggestions, ideas will be greatly appreciated

D

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'92 75 HP Mercury 3 cylinder runs half speed 1 month 6 days ago #137291

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Took my engine to a highly regarded repair site in northern Michigan, Pioneer Hills Marine is Roscommon, MI. After describing the symptoms to the service rep, he suspected a faulty stator. He explained that the stator in this engine has both low speed and high speed windings and is designed such that as the engine accelerates control of the spark transfers from the low to the high speed winding resulting in a smooth transition in engine speed. He further explained that if the high speed winding for some reason fails, the engine will only accelerate to half speed. Because the throttle is fully open, each cylinder receives a full charge of air/fuel but too weak a spark to burn the whole charge. Thus, the engine runs smoothly but too rich. This seemed to describe the performance I experienced. I left the engine to be tested with the center's diagnostic tools and will report on the resulting diagnosis and repair.

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'92 75 HP Mercury 3 cylinder runs half speed 1 month 4 days ago #137329

He told you the truth.
Hopefully he nailed it for you.
doc

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'92 75 HP Mercury 3 cylinder runs half speed 4 weeks 1 hour ago #137425

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I don't have her back yet, but the repair tech says the middle coil pack was not firing and needed to be replaced. The original theory of faulty high speed stator wiring seemed so logical, but I hope for cost reasons that the coil pack replacement solves the problem.

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'92 75 HP Mercury 3 cylinder runs half speed 3 weeks 2 days ago #137451

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Coil pack was the problem. Replaced it and she purrs like a kitten. Lucky me!

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'92 75 HP Mercury 3 cylinder runs half speed 3 weeks 1 day ago #137452

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Ya got a great mechanic! Honest, knows his stuff!

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