1956 Johnson 15HP ....no power ???

[Wishiniwuzfishin]

Just bought a 1956 Johnson 15 hp. Ran it in driveway in water tank and seemed to run good... spit water good. Put it on my 12 ft boat and took it out and again seemed to run good but just wouldn't run at any speed. Fastest reading I got on my gps was about 7 mph. I previously had a 9.9 on the same boat and I was going 22mph according to gps with the 9.9. Thought I'd really fly with the 15 but nope. The 15 started ok, idled ok, throttled up and down ok. Everything seemed ok just wouldn't go fast !!! I really can't say how many rpm's it was turning but I could definetely hear it throttle up and down. I was using the same gas tank with the same fresh gas and oil as the 9.9 so that wasn't the problem. My question is, after doing some searching on here it sounds like it may have only been running on one cylinder. Is it possible for the motor to only run on 1 cylinder and behave as a perfectly running motor except for the top speed? If I wouldn't have known it was a 15 hp motor I would have guessed it was a great running 3 hp motor on the boat!! The weather here is really nasty right now and it may be a few days before I can work on it, but I guess when I get the chance my first step will be to pull the wires off 1 cylinder at a time and see what happens. Then I guess a carb clean and rebuild would be my next step. Any help will be greatly appreciated.

 

[restornator]

Re: 1956 Johnson 15HP ....no power ???

You will be amazed at how well (not really) they run on one cylinder. It usually points to other causes because the symptons are indentical; won't idle down, sputters, weak power, hard starting, etc.

The way you describe it, there aren't any symptoms like the ones above. Are you sure you are getting WOT? (the throttle cam is reaching it's maximum movement?)

 

[Wishiniwuzfishin]

Re: 1956 Johnson 15HP ....no power ???

Yes. Forgot to add a few things. When I got back I compared some things to the great running 9.9 (also a 1956). The throttle connections moved the exact same. The 15HP hit the stops correctly for neutral, reverse and forward. The throttle connections rotated all the way...maybe even a hair farther than the 9.9. Everything seemed to compare ok visually with the 9.9. These are the connections from the throttle control in the powerhead. Haven't pulled the air silencer yet to look in the carb. Also, while running in the driveway I checked compression and both were 75 but I think I may have forgot to reset the dial after doing the first cylinder and before doing the second, so I can only say 1 cylinder is good. For reasons I won't go into, my compression gauge fitting wasn't getting a perfectly air tight seal in the spark plug hole so I was happy with the compression readings.

 

[Chinewalker]

Re: 1956 Johnson 15HP ....no power ???

I would check to see if it is running on one cylinder. Do it by removing one plug wire and starting the motor. If it starts and runs as it did before, then the cylinder with the pulled wire it not running. If it does not run at all, then the pulled one is the "good" one.

Could be poor spark. Could be blown head gasket (although not likely with your compression readings). Could be a few other things, but we'll get into those only if needed....

 

[CATransplant]

Re: 1956 Johnson 15HP ....no power ???

It's most likely no spark on one cylinder. Unless the coils have been replaced, at least one is probably bad. That should be the first thing you check at this point. In fact, with your symptoms, it's almost certain that one plug isn't firing.

 

[CATransplant]

Re: 1956 Johnson 15HP ....no power ???

Are you sure that other outboard is a 1956 9.9? They made a 10 hp then, but not a 9.9.

 

[Wishiniwuzfishin]

Re: 1956 Johnson 15HP ....no power ???

Sorry, it is a 1956 10hp. Has the round 10 emblem on the cover. We have a 9.9 hp limit lake around me (at least it used to be) and 9.9's are extremely popular around here. I somehow got used to referring to all 9.5's, 9.9's and 10's as 9.9's. I haven't really checked anything besides the compression yet, and I do need to recheck it first. Like I said I'm pretty sure I forgot to reset the dial in between cylinders. Going to try pulling the wires 1 at a time after that and see what happens. If I have good spark in both cylinders I'll look at the carb I guess. The thing that just keeps puzzling me is how good it ran except for the slow top speed. Have to wait for the rain and wind to stop before I get out to do anything to it. Thanks for all the replies so far.

 

[asdasc]

Re: 1956 Johnson 15HP ....no power ???

Yeah, I had my '59 18hp running for quite a while on one cylinder. I didn't know, and had completely tuned the carb to run really smoothly and start really easily on one cylinder. It wasn't until the good cylinder failed and I couldn't get it started when I realized I was running on one all that time. I just thought I had too little motor for my pontoon boat.

What a difference it made to run on both! Still not enough motor, but MUCH better.

 

[Wishiniwuzfishin]

Re: 1956 Johnson 15HP ....no power ???

Got a chance to work on the motor. Checked for spark....only getting spark in 1 cylinder. I am amazed that the motor can run so good on just 1 cylinder. Also checked the compression again, making sure to reset gauge between cylinders....80 lbs in each cylinder with a cold motor and I still don't think the fitting in the spark plug hole is sealing as good as it could. Guess it's time to start troubleshooting the ignition. Gonna pull the starter and have a look at things. Probably just go ahead and buy all new points, condensers, coils and wires since the motor seems so good otherwise. Does anybody know if I can use my dwell meter I use on my old 67 Chevy truck to set the points on this motor? Never was good at doing them with a feeler gauge. Not sure but I think the meter has settings for 4, 6, and 8 cyl. Not sure about 2 cyl.

By the way the problem with my compression gauge is the fitting for the spark plug hole is too long and screws way too far into the cylinder. The first time I tried it the piston hit the end of the fitting and wouldn't turn over. I kinda thought it was gonna do that so I didn't pull the starter rope real hard. I have no idea why the fitting is so long. Anyway, I bought some rubber o-rings to put on the fitting so it wouldn't srew in so far. Ended up using 5 o-rings but now if I try to tighten it past just barely finger tight the o-rings start to distort. I think I might need smaller diameter o-rings. But the fitting is sealing much better now. Before I got the o-rings I just backed the fitting out so the cylinder wouldn't hit it and the fitting would wiggle in the spark plug hole so I know it wasn't sealing good.

 

[Hanr3]

Re: 1956 Johnson 15HP ....no power ???

Did you get this fixed?

 

[Wishiniwuzfishin]

Re: 1956 Johnson 15HP ....no power ???

Not yet. Finally ordered all ignition parts for it recently but not long after I made this post I got involved with another boat and 2 motors (1974 115HP Johnson and 1968 9.5 Johnson) I have. Probably won't get back to this one till I put both my boats away for winter.