75HP Johnson......not running....need help

[jdwilliams1]

It is a 75HP Johnson and 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, compression is 85-80-85-85, has good gas

It has run recently so the previous owner said.

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

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.

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

 

[coueytwo]

it's an 1963 model, you have a distributor on it, if the wires are run to the wrong spark plugs it will back fire.

 

[witenite0560]

I've got a similar problem with my 1960 75hp Johnson. I've made sure the spark plug wires are correct. Have checked the timing on the magneto several times, as well as magneto/carb synchronization. All 4 cylinders have 70-75 psi when hand cranking. Once in a while she'll backfire once when starting. But, generally starts after a bit of cranking. Just not terribly satisfied with idle smoothness, seems to hesitate/miss a bit when accelerating and can't get more than 4200 rpm (that may be a prop issue, of course). I kind of wonder if the somewhat arbitrary timing of the magneto isn't off one tooth of the belt. I say "arbitrary" because the book says you kind of eyeball the marks and depending on what angle you look from, it could be off quite a ways.
I'd appreciate any suggestions as well. Thanks.

I did file the points, but have not replaced them. I think my mixture still needs a bit of tweeking, too.