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20 hp merc still runs bad

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Saasman1163

New member
Ok...when I run this motor at wide open it runs great for a while then starts to rev wierd and bog down until it quits. If I let off the gas and take it to low speed it seems to run fine. I can run at low speed for a while and take it to wide open for a half a minute or so and it does it all over again. It also has a rough idle as well. Do I start with the carb first? Or could it be electrical. It is a late 70's early 80's motor. Any ideas? Also who knows of a good repair guy in the northern suburbs of minnesota. Thanks
 
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pump the bulb manually and see if the problem goes away while running at max, could be sucking air. Adjust idle mixture screw for smooth operation while in gear
 
I tired pumping the bulb and it worked on the old tank, but not on the new one. The vent cap was open as far as it can be, but it runs even worse with the new tank.
 
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