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

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

Re: 75HP Johnson......not running....need help please

sounds like the guy before did not have the right firing order check first to see if the wires are on the right plug also what year motor

Re: 75HP Johnson......not running....need help please

It could be the timing, Check and make sure that you don't have a sheared key on your crankshaft letting the flywheel slip.

Re: 75HP Johnson......not running....need help please

I think the firing order is good but I am basing it on the numbers written on the cylinders.

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

Sorry for not mentioning that before.

Your help is appreciated. I just mentioned the sheared key to my dad and that might make sense.

Re: 75HP Johnson......not running....need help please

sturgis said:

It could be the timing, Check and make sure that you don't have a sheared key on your crankshaft letting the flywheel slip.

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OK, so is this the key in the crankshaft?

Re: 75HP Johnson......not running....need help please

jdwilliams1 said:

OK, so is this the key in the crankshaft?

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It better be! You said earlier that the previous owner replaced the magneto, in order to do that he had to pull the flywheel, the flywheel slides down over the half moon key that is in the slot on the crankshaft. If that key gets tilted out at the top the flywheel can push it out insteed of sliding over it. Also if the installer forgets to tighten the flywheel before starting the engine it will work loose and sheer the key off. Only way to check is pull the flywheel, you might get lucky and it will pop right off but in all probability you will need a wheel puller. Good luck!

Re: 75HP Johnson......not running....need help please

If the head covers have ever been replaced, then perhaps they could have been switched port-side to starboard-side. Therefore the numbering cast into the head covers will be incorrect. The numbering on your motor is top starboard side 1, lower starboard side 3; top port side 2, lower port side 4. This is unlikely your problem, but check to be sure because it really sounds like the firing order is off.

Re: 75HP Johnson......not running....need help please

Did you ever get this Running?

Re: 75HP Johnson......not running....need help please

You know I have the motor running now but cannot get it dialed in all the way. I can get it to idle great and it will get to top speed eventually. there is a good deal of sputtering as it tries to reach top speed. Once I get to top speed I can back of to mid speed and then it re-accelerates very nicely back to top speed. Now I have broke the carb down 3 times and cleaned as best I know how. Carb cleaner, then blown out with air compressor etc.

There is also what appears to be a lot of fuel behind the motor as I idle and then when I hook the boat to the trailer I always leave a good deal of fuel right at the ramp ( the ramp is steep though.

I cannot figure this thing out, I have made adjustments to the carb while on the water and if I lean things up it dies and wont idle or start and if I rich it dies.

When I first accelerate is hesitates and then speeds up like it grabs some fuel and or burns off enough for it to run right, then there is another hesitation and the same thing repeats until it finally get to top speed.

Any thoughts

sturgis said:

Did you ever get this Running?

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Re: 75HP Johnson......not running....need help please

2 thoughts come to mind...float level and drop...inspect reeds for any damage....were thes carbs rebuilt or just cleaned?

Re: 75HP Johnson......not running....need help please

I rebuilt them with a kit, put in a new float and I assume the drop is the pin that goes down in the hole under the float?

atlapp said:

2 thoughts come to mind...float level and drop...inspect reeds for any damage....were thes carbs rebuilt or just cleaned?

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