1977 75 Johnson head scratcher

niegelp

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Jul 23, 2010
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Been working on a '77 75hp Johnson motor.


Its hard to start, when I can get it to run it will idle for a while, then slowly drop RPMs til it dies. Almost like it is flooding out. When I take it on the river, its even worse. Sometimes it will die and not restart at all. When I get it to run enough to take it out, about 4,300rpm is all it will handle. And that's at 3/4 throttle. When at full throttle it almost seems like its pegging off the rev limiter.


First checked compression and all three are even at 95psi; a little low, but even. Did a leak down test, with 0 - 10% leak on all cylinders; there again, even. I did the link/sync procedure, making sure my throttle isn't opening too far. The timer base tested bad so I have replaced that, along with the power pack. I cleaned the carbs, rebuilt the fuel pump, and even tried new reeds with no improvement. The last thing I did was pull the head to visually inspect the cylinders, which all were fine, no scoring. The one thing I did find in the cylinders was a brown, thick oil like substance similar to unburnt oil. The motor isn't overheating or anything like that, running a nice temperature.


I am at a complete loss on this one, anything anybody has in mind will help.

Thanks
 

Bosunsmate

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Apr 7, 2012
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Re: 1977 75 Johnson head scratcher

Have you tried pulling each plug in turn to check if any arent taking much or any load?
Have you tried keeping motor off and prime the bulb hard and check it holds pressure?
 

niegelp

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Jul 23, 2010
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Re: 1977 75 Johnson head scratcher

Have you tried pulling each plug in turn to check if any arent taking much or any load?
Have you tried keeping motor off and prime the bulb hard and check it holds pressure?


I have tried pulling each plug, each cylinder drops when you remove each plug wire individually.

Have not seen if it holds pressure. But when I pulled the reeds out they were all full of unspent fuel. Never seen that before.
 

Bosunsmate

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Apr 7, 2012
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Re: 1977 75 Johnson head scratcher

check if it holds pressure then
have you a choke or primer on that motor?
 

niegelp

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Jul 23, 2010
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Re: 1977 75 Johnson head scratcher

Yeah, I tried choking it while running but only brought the RPMs down faster. I thought maybe it was starving for fuel but the plugs are wet after I do a high speed shut down. Almost like it's getting more fuel than it can burn off.
 

Daviet

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Sep 24, 2008
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Re: 1977 75 Johnson head scratcher

Couple of quick questions.
When you overhauled the carbs, did you get the idle orifice screwed in tight and the plug over it tight?
Is the little round gasket in position, the one you can see through the middle of the float?
Real silly question, are the floats installed properly and set to the correct level?
 

niegelp

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Jul 23, 2010
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Re: 1977 75 Johnson head scratcher

Yeah, everything is tight in the carbs.

I replaced the cork gaskets with the new rubber or whatever they're made of. Float height is set, just the same that I have set every motor I have worked on from that era.

Its funny because I had another 75 horse in the shop that ran like a top; although the compression on that motor was a lot higher, 130 or so. I'm almost thinking that even though the compression is even, it may be too low for proper operation.
 

niegelp

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Jul 23, 2010
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Re: 1977 75 Johnson head scratcher

Alright, after a bunch of screwing around, I checked the bottom of the cylinders. Looking thru the reed plate area, I could see some pretty substantial scoring on the bottom half of the cylinders. I'm guessing this beast hasn't been overhauled since new.



Thanks for all of your ideas and whatnot.
 
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