I have a 1993 70 Hp Johnson that runs great for the first few minutes at full throttle, but then begins to fade. I notice that it starts to slow down so I turn back to the dock and it dies at the dock. After looking online, I decided to replaced all the fuel lines, put in a fuel/moisture separator, and replaced the power pack. A friend of mine and I pulled the spark plugs out and cranked it over and noticed that the top two cylinders were not “spitting” fuel out the spark plug holes. I drained the bowls to the top two carbs and re-primed the carbs. The motor started a little rough, but eventually got started. I took it back out today for a sea trial and it started up great; however, after a few minutes at full throttle, it did exactly what it did last time. At full throttle it started to fade so I turned back to the dock, and by the time I made it to the dock, the motor cut off.
Any thoughts?
Thanks for your help!
Maybe the tank vent.
‘87 Pacemaker 31’ SF
‘04 Renegade 29’ 2 225’s
‘97 Maycraft 17’ 40 hp
My next step would be to check your ignition coils, they might be breaking down when they get hot. I think the procedure is in the seloc manual
pump the primer bulb when it starts to fade and see what happens
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Just got the compression tested and great news as follows:
Top cylinder: 120 psig
Middle cylinder: 35 psig
Bottom cylinder: 90 psig
I think the fading occurred because as the powerhead thermally expanded after running for a few minutes under load, the middle and bottom cylinders really lost their compression. Next question is can I replace the rings since the engine is not locked up or does this require a new powerhead? What are you thoughts?
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Tear into it and take a look. Could be a blown head gasket, cracked head, hole in piston, cracked piston, scarred cylinder wall, or broken rings. Sounds like it could be a head gasket with two cylinders with low compression beside each other.
‘87 Pacemaker 31’ SF
‘04 Renegade 29’ 2 225’s
‘97 Maycraft 17’ 40 hp
The problem might not be the rings, I’d first suspect the head gasket, but that’s an inexperienced opinion. You may be losing compression between the middle and bottom cylinder.
If you are doing this yourself, you will probably be able to see the rings just fine when you pull the head off.
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