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Mercury 700 low power

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agreszler

New member
I'm new to this forum. I have an old Mercury 700 (3 cylinder 70 HP). It is idling rough and has very low power output. When idling, I can remove the center cylinder plug wire with no discernible change. If I remove the bottom plug wire, the idle is rougher. If I remove the top plug wire, the engine dies. I found the center plug is very wet when removed and doesn't show any carbon which would imply no combustion. I checked the spark in the center cylinder and it seems strong enough. I ran the engine briefly with the center plug removed and it seems to be spitting a lot of fuel from that cylinder. According to my Seloc manual, excess fuel in one cylinder could be caused by a leak in the fuel pump diaphragm. I removed the fuel pump and did not see an obvious leak, but, of course tore the gasket upon removal. I have ordered the gaskets and diaphragm for the pump. I'm not confident that will fix it and I'm looking for more ideas. Thanks.
 
racerone said:
Compression test should be done.
Click to expand...

Top cylinder checks at 130 PSI, middle at 125, and bottom at 150. On second look, I suspect what is spitting out of the center cylinder is a fuel water mixture. Now, I'm suspicious that the gasket for the fuel pump was leaking allowing water into the area behind the diaphragm. Does that make sense?
 
Jeff, I replaced the fuel pump gaskets and diaphragm with no improvement. I suspect you are correct that block is cracked or perhaps corroded through to the cylinder. I'm not sure it's worth trying to repair this old motor.
Thanks,
Tony
 
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