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#12328057 - 07/07/17 10:10 AM mid 50's 7 1/2 horse johnson motor
craiglane24 Offline
Green Horn

Registered: 07/07/17
Posts: 1
I have recently come into a old jon boat with that has a 7 1/2 horse johnson motor on it. Looking it up it is a 54,55,or 56 motor. I am wanting to change the lower unit oil and check the plugs and general condition of the motor before I try and fire it up and have something go wrong.

Have anyone had any experience with this that can give me pointers?

#12330001 - 07/08/17 04:32 PM Re: mid 50's 7 1/2 horse johnson motor [Re: craiglane24]
IslandJim Offline

Registered: 07/08/14
Posts: 369
Loc: Rockport, TX
I went through the same scenario last year with a 1961 5.5 Johnson Twin. Great engines. Yours is basically the same style as mine, but about 1960, they went from the round case to a '60's "tail-fin" look like the cars had.
Several things I learned: To drop the lower unit(which you should do) and change the water pump impeller AND the fat "O"-ring seal around the shift rod, you have to pull the power head to disconnect the shift rod. A major pain in-the ^ss! To replace the shifter rod seal, you have to get the brass seat out. Videos on the Internet for that. If you don't change that, you WILL get water into the lower unit. Most of the parts you will need are available through
If yours is a '56, it's an AD-10, '57 is AD-11, '58 is AD-12. But calls the '57 a Model 7522.

I fought a problem on mine with running on one cylinder intermittently, until I took out the points and condensers and replaced them with two electronic sensor modules. Mine were made for Kawasaki, but there's a lot on the market. About $20. Those solved my problems immediately. Now, my 56-year-old motor runs like a top.
Goog luck! IslandJim
I'm an Eighth Day Adventist. On the Eighth Day, God went fishing!

#12330666 - 07/09/17 06:43 AM Re: mid 50's 7 1/2 horse johnson motor [Re: craiglane24]
sbump26 Offline
Extreme Angler

Registered: 03/10/04
Posts: 1938
Loc: Arlington, TX
I learned a lot from when I bought a 55 10HP Johnson. Great bunch of folks that know these outboards like the back of their hand and willing to help. I left mine original with the pressure tank. Very easy to start once the carb was rebuilt, points and condenser replaced and set, and the timing linkage repaired. Great time learning and getting it running. Would start half way through the first pull on the rope.
Put the plug in.

#12330838 - 07/09/17 09:48 AM Re: mid 50's 7 1/2 horse johnson motor [Re: craiglane24]
lenahorse Offline
TFF Team Angler

Registered: 10/11/12
Posts: 4091
Loc: Cash, Texas
I grew up with a 7.5 sea horse, could you post a pic of it?
Fishing on Texoma

#12332738 - 07/10/17 04:02 PM Re: mid 50's 7 1/2 horse johnson motor [Re: craiglane24]
redchevy Offline
TFF Celebrity

Registered: 01/25/05
Posts: 7168
Loc: texas
I have a 1957 7.5

I haven't used it in a few years now. I would change the lower unit lube, if it pumps I wouldn't necessarily worry about the water pump, mine sat up for a decade or more before I got it going again and I ended up changing it after using it for a few years and it looked fine, it is a pretty robust design.

If you didn't know make sure to mix your fuel/oil 16:1.


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