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1978 20hp ......Need Help

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I have a problem making me nuts ! This is a late 78 model that runs great at all speeds and starts on the first or second pull no problem. After running under load for about 10 min and then slowing down it starts running really bad. Will barely keep running at all. I have rebuilt the carb and fuel pump,put new plugs in it and run off two different tanks and lines.....same problem. When I first start it I have about a 10 min slow run to get to fast water and it is fine, but after a few runs at speed just a few minutes this problem occurs and I barely make it back to the dock. At half throttle it is just barely running and not very well.
It is not getting hot, water is just warm, bulb is still hard and sometimes it will catch and speed up or sometimes just die and not want to restart.

I am not fond of rowing Any ideas ?
 
Sounds like overheating electronics. Try shooting the switchbox with some CO2 from an extinguisher when it does this, to cool it, and see if it picks up.

Jeff
 
You can pickup a can or two of "Freeze Spray" at an electronics store (I think this has not been banned ?) we used to use it on computer chips.

IS your FLYWHEEL getting hot - esp at the nut ???

Also get some starting fluid and when it starts to die - spray in the carb(s). If it pick up RPM it points to lack of fuel. Otherwise I'd look for a temp sensitive component (Coil insulation rubbing when expanding?) etc.
 
I have a can of freeze spray and will give it a try this weekend and check fly wheel temp.I have a heat lazer type sensor as well. I really thought it was a fuel issue but seems strange that the problem always occurs at about the same amount of run time. I appreciate all the ideas it really helps ! Will report back this weekend, I am on the water every weekend.
 
Have a 2001 Mercury 60hp 4 stroke 4 cyl. I did a full cleaning of the carburetors. I pulled the the butterflies to dip the carbs. When I reinstalled them I thought they were all the same but finding out when I started the motor back up they are not. Nothing in the service manual about the butterflies besides the adjustment.
Please help motor setting still until I can figure it out!
 
I just bought this motor and was told he had just replaced the impeller so ??? not really sure. It seems to be pumping fine and the water temp coming out at the time of failure is just luke warm.
 
I was reading the manual last night and I believe it has the wrong spark plugs installed. It had ngk plugs in it and I replaced them with the same. But the cross from the champion number in the manual to an ngk plug is different then what is in there.......Could this cause the problem ??
 
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