72 Johnson 50HP Hard Cranking

schwar74

Cadet
Joined
Nov 28, 2010
Messages
17
Greetings,

I bought a boat that came with one of these motors. I went with the deal primarily for the hull and troller, but thought I'd see if I could resell this motor as a runner.

Well, I've run into a couple snags from the get go.

I can't get the motor to crank over with any speed. I thought initially it was the starter, so I bought what was supposed to be a good used one for $40, but its acting the same. The motor cranks, but it seems to have alot of resistance. When I pull the plugs, the motor cranks over at a normal speed. There's lots of air that gets pushed out of the cylinder with the plugs out. When the plugs go back it, its right back to this slow cranking.

The other thing that doesn't seem right is that I hear a hum in the exhaust housing when the key is in the on position. I thought maybe there was a short in there that was draining voltage away from the starter, so I disconnected the wire from the starter and jumped it directly from the battery which was connected to a charger/booster. I'm pretty sure that has gotten enough voltage to the starter.

Is there a reason for the motor to "over-compress"? Is it possible there's something lodged in the exhaust thats putting too much pressure on the engine?

One other thing, my understanding is that the voltage that serves the shifting solenoids comes from the internal electrical system, and not from the battery. When the motor cranks with the plugs out and the flywheel actually spins with some rpms, the prop does not turn, so I'm taking that to mean that the shifting system is not the problem. In other words, its not trying to start the motor and turn the prop at the same time, thus overloading the starter.

I'm not that "into" this motor so I don't really want to go overboard with it. Plus, I've read all the bad opinions of the hydro-elec shifting system. Any suggestions?

Thanks,

Scott
 

jbjennings

Captain
Joined
Jul 18, 2007
Messages
3,903
Re: 72 Johnson 50HP Hard Cranking

When you say it cranks, do you mean that it runs? Or, just turns over?
If it turns over slowly, try reversing the spark plug wires and see if it fires up and runs. If they're reversed, it will of course be out of time and be extremely hard to turn over.
Other than that, bad connections or bad wiring, or bad starter.
Best guess,
JBJ
 
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