TOPIC: 75HP Johnson....I think...not running....need help

My plans are to learn as much as a can about these motors. I was told it was a '63 but I am going to verify, because I think the boat it a '64(doesn't mean motor could not be a '63)



It is a 75HP Johnson and will I was trying to fire it up yesterday it turns over and then after a couple seconds there is a loud pop...almost a backfire sound. I have put new plugs on it I will get a spark tester and compression checker...but any thoughts?

It has run recently so the previous owner said.

It is a Johnson Super Sea Horse, Electric, I think 75HP
V4S 15R
2385713

Thanks to all you who are much smarter than I....I am getting more knowledgeable every day

1963. Try opening the carb jets 1/2 turn.

I believe Andgott has the manual for that motor posted on his website;
www.agott.com/glassboat/manuals.html
Have you covered the basics - fresh gas, fresh spark plugs, etc.?

First do a compression test.

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Check carburetor for cleanliness.

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Take note of the timing marks.

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Remove magneto

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clean / repace points, replace condenser. Have coil tested with a MERC-O-TRONIC ignition analyzer.----Inspect / replace impeller.---Drain and refill gearcase.

ok, I have put in new plugs, new gas, the compression seems ok, however I do not know what would be acceptable numbers, they are all withing 10% of each other 85-80-85-85.

After a crank for awhile I get a loud backfire, smoke from the exhaust and it will not start. Would this be a timing issue possibly?

How would you proceed? I was told from the guy I bought it from that the motor did run, but he replaced the magneto and I am starting to wonder if that did not get back together the right way.

Please help.

try squarting some gas in the carb and see if it will run for a few seconds. If it fires its a fuel problem.
Mike aka pathfinderz1

Pull the plugs, are they wet? If so and since it does fire once in a while, it does seem to have spark and fuel. Compression is enough to run. So my guess is timing will be found to be off.

sounds to me like the fireing order maybe incorrect, that would be my next ck