1981 75 HP Johnson Seahorse

Forty Six and 2

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Jun 14, 2009
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I just bought this motor and mounted it, started it up and ran for about 20 mins, died and will not start again. The cranking of the motor is very slow. I replaced the starter and solenoid. Have a brand new battery with 650 CCA and all new wiring. Fuel is being delivered but it seems the starter can barely crank the motor over? Tried checking compression and get this 120 / 120 / 105. Hard to tell on the top cylinder as that is when the motor decides to rotate slowly. Any suggestions?
 

shipwreck

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Nov 23, 2006
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263
Re: 1981 75 HP Johnson Seahorse

You replaced the starter with new one?
I would take spark plugs out , ground the ign system rotate the fly wheel by hand and see if the pistons turn free or are hang up. shine flashlight in hole make sure your piston are moving to tdc and back. Then take a non sharp object insert in each plus hole , hold the flywheel with one hand and push on piston- it should not move if dose could be loose connecting rod.

If top checks out you could have something in lower unit wrong, have checked gear oil?
 

Forty Six and 2

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Re: 1981 75 HP Johnson Seahorse

I drained the lower unit. It was a bit low and somewhat gray. I put all new fluid in it. Will try to rotate with plugs out. Any other suggestions? The starter has been replaced.
 

shipwreck

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Nov 23, 2006
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263
Re: 1981 75 HP Johnson Seahorse

squirt some oil that mix with gas into the cylinders and work the flywheel see if start better , i would think the piston hang up or blown head gasket.
The motor had this problem with old starter and solenoid?
That 3 cylinder i think the 105 psi cyliber is what i would check first.
 

shipwreck

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Re: 1981 75 HP Johnson Seahorse

checking to see what found out if had time?
 

jtexas

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Oct 13, 2003
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8,646
Re: 1981 75 HP Johnson Seahorse

120 is fine for that vintage, but 105 is too low compared to #1 & 2. That alone doesn't account for your symptoms, though.

Are you getting 12V or more to the starter? Measure from starter terminal to battery (-) post, key turned to "start".

If so, I'd be inclined to pull the cylinder head for inspection. Highly recommend you acquire the factory service manual.
 

Forty Six and 2

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Re: 1981 75 HP Johnson Seahorse

I do have 12 volt at the starter. Just picked up Seloc manual and going to pull power head. Have to wait a few days now. Duty calls at work, wish we could all go boating and fishing everyday. If the # 1 is to low what do you think next steps may be?
 
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