1980 Johnson 75hp Stinger Power Problems

Looking for some ideas. I am new to boat ownership, so I bought myself a 1978 Sylvan Sportsman 16' V-Bottom Aluminum fishing boat with a 75hp Johnson Stinger. Boat is in great shape so I thought it would be a nice first boat for me and my young family. Took the boat out a few times after purchase and had problems starting and had a hard time to keep it running. Motor would die at idle and when putting into gear, but once you kept it running and got to full throttle, the boat got out of the water quick and moved really good. It would run well over 6000 rpm's. Since then we have put in new plugs, cleaned and rebuilt carbs, added a bottle of seafoam to the tank to run through the motor, tried different sizes of jets, put a new prop on to bring down the rpm's (went from a 17 pitch that was on it to a 19 pitch), and replaced batteries. After all of this my starting issues have gone away and my dieing issue has gotten better. My problem now, I have no power. At full throttle I am only getting 1600 to 1800 rpm's and I can't go any faster than trolling speed. When I take the cover off the motor I seem to have a lot of gas and oil that is not burning up. I have had some suggestions of bad coils (mine don't look burnt and seem to have been replaced recently), a new stater (how would i test if mine is bad before i spend $207 for a new one), plugged exhaust, low compresion (mine measures 105 to 110).

I have called some boat mechanics in my area and most of them do not work on motors of that age. So it looks like I'm going at this alone.

Any suggestions would be greatly appreciated.

Re: 1980 Johnson 75hp Stinger Power Problems

correct me if I wrong, but isn't a 19 degree pitch more of a load and made for speed than a 17 degree? This could be why you have limited power. I had a 17 and went to a 15, more power, less speed.

Re: 1980 Johnson 75hp Stinger Power Problems

Pitch has nothing to do with degrees. It is the number of inches that a propeller would theoretically move through a solid medium in one revolution. That said, the OP mentioned his initial RPM was over 6000, which is too high. He went up in pitch (akin to shifting from 3rd to 4th gear) to bring RPM down a bit.

You're due for some basic troubleshooting. Compression numbers sound on the low side. I have the same motor and mine punches my gauge into the 140 neighborhood on all three slugs. Could just mean you have a wonky gauge, but you should still be running better than 1800 revs. Check spark on all three cylinders. You mentioned excessive fuel inside cover. Check hoses for leaks. Possible one or more carbs isn't getting fuel. You could fire motor up on muffs and try spraying some mixed fuel from a squirt bottle into each carb to see if it picks up a cylinder. Does the squeeze bulb in the fuel line get hard when primed? Does fuel come out any of the carbs (motor off) when priming bulb? What do your spark plugs look like?

Let us know what you find...

Re: 1980 Johnson 75hp Stinger Power Problems

Chinewalker said:

Pitch has nothing to do with degrees. It is the number of inches that a propeller would theoretically move through a solid medium in one revolution. That said, the OP mentioned his initial RPM was over 6000, which is too high. He went up in pitch (akin to shifting from 3rd to 4th gear) to bring RPM down a bit.

You're due for some basic troubleshooting. Compression numbers sound on the low side. I have the same motor and mine punches my gauge into the 140 neighborhood on all three slugs. Could just mean you have a wonky gauge, but you should still be running better than 1800 revs. Check spark on all three cylinders. You mentioned excessive fuel inside cover. Check hoses for leaks. Possible one or more carbs isn't getting fuel. You could fire motor up on muffs and try spraying some mixed fuel from a squirt bottle into each carb to see if it picks up a cylinder. Does the squeeze bulb in the fuel line get hard when primed? Does fuel come out any of the carbs (motor off) when priming bulb? What do your spark plugs look like?

Let us know what you find...

Click to expand...

Thanks for all the info. Shortly after my original post, I ran out of ideas so I contacted a mechanic here in Ft Collins, CO for some advice. I wound up taking my boat to him for a trial run. With a little work, he came up with my jets being wrong for the altitude along with my prop. Since, we have made the changes and the boat is running great and catching fish. I went to a 15 pitch prop from the 19 pitch that was recommended to me by another boat parts house.

I do have another motor that I am working on now. My grandfather had a 1956 5hp SeaKing on his boat when I was a kid. He passed a few years ago and I was fortunate to receive his boat motor from my Grandmother. The motor has not been in the water for probably a good 15 years. I put gas in it, new plugs, and pulled the chord and it fired right up. While running, it seems to speed up and slow down on its own. After about an hour of running it will cut out and die, when trying to restart it will not run longer than a minute or two. I'm eager to do some work on this motor, but it will probably have to wait until this winter when I will have some more time. If anyone is familiar with this motor and has any advice for it, it would be greatly appreciated.