1975 75hp Johnson - Warped head? Blown gasket?

bloopbloob

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Jul 27, 2011
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Engine overheated first time i ever took this boat out. Had the water pump replaced, mechanic said my head was warped and motor is toast. Took it out again anyways, ran 2 tanks of gas through it pulling a tube, and for the most part, it ran pretty well, except a little sluggish sometimes. Doesn't overheat anymore, cool to the touch. It now runs cool, but believe that the middle cylinder is dead. Spark plug has grey oil/water mix on it. I know very little about boat engines, but am fairly handy, to a point. What should I be looking for first. How hard is it to take it apart and replace a gasket and see if that helps? Thanks!
 

Exotic4x4

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Re: 1975 75hp Johnson - Warped head? Blown gasket?

Do a spark test on that cylinder, also compression test I would strongly doubt u warped the head on that head gasket is possible but a spark test would show you if that cylinder was burning the fuel instead of throwing all the gas out of the exhaust also a warped head doesn't mean its a bad head . You can get them magnafluxed for cracks and re surfaced for 150$ hope some of this helps
 

bloopbloob

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Re: 1975 75hp Johnson - Warped head? Blown gasket?

All info helps! Thanks! Mechanic ran it in his shop after swapping the waterpump, said it was still running hot. he then pulled plugs and said #2 cylinder was full of water, so that meant the head was warped and not worth fixing, so drive it till it dies.... So I took it out, and i could hold my hand on anything for as long as I wanted, barely warm. Pulled a tube for 3 hours. If compression test fails, how difficult is it to get to the gasket and take a look?
 

Daviet

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Re: 1975 75hp Johnson - Warped head? Blown gasket?

Those heads come off pretty easy. Parts are available here, that engine is worth repairing even if you have to get a used cylinder head.
 

Exotic4x4

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Re: 1975 75hp Johnson - Warped head? Blown gasket?

Generally if heads are warped depending how bad they are junk is find a reman head or a good used one but if its water for sure I'm your cylinder you could just have a bad head gasket like i said take it off get it magnafluxed for cracks and what.not see if it can be milled flat..
 

bloopbloob

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Re: 1975 75hp Johnson - Warped head? Blown gasket?

So I did a compression test, did it without warming up the engine because i don't want to use muffs (brand new water pump, and heard muffs are very hard on them), and in neutral ( not WOT like i've heard because it won't start in gear). top cylinder read 138, bottom cylinder 138. Was unable to get a reading from the middle cylinder (the one with water in it) because the beginning of the threads have been cross threaded, i just discovered. In the process, i discovered a spark coming from a crack in the top coil assembly (a crack in the blue outer housing) part #46 here75 johnson 3.jpg , I believe.

SO a few questions..... What can I do about the cross threading? Do I have to replace all 3 coils? Difficult job? And I think i'm just going to go ahead and try to replace the head gasket, what all has to be removed? Do I have to take the cylinder head cover off the cylinder head, (#15 and #13 here) 75johnson.jpg? Just wondering what gaskets I'll need, and anything else I may not know about? Thanks!
 

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Haffiman

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Re: 1975 75hp Johnson - Warped head? Blown gasket?

Pull the head and have it checked for 'warp'. If not to bad, it may be planned out using a 800-1000grid wet-paper glued to a 1/2' thick piece of glass.
Replace cracked coils, you may take only one, or your powerpack will soon be gone.
As for the spark-plug thread, sometimes it may be 'rescued' by tapping new thread from the inside using the fresh one as entering threads. Easy when head is off.
No need taking the cover off, but might be smart to check on thermostat when head is off.

PS:
Change mechanic at the same time!
 

Haffiman

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Dec 17, 2009
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2,454
Re: 1975 75hp Johnson - Warped head? Blown gasket?

If you have sparks from the secondary of your coil, around 20Kv, 'jumping' around from a crack, it may find its way into the primary from the power pack. I will have the same effect as running 220/110Volts into your USB port on your computer!
 
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