Topic: 1975 Johnson 15R75C 15 hp – Page 3 – Antique Outboard Motor Club,Inc

This topic has 33 replies, 8 voices, and was last updated 1 month, 2 weeks ago by Buccaneer.

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August 30, 2025 at 8:41 am #299427
David Bartlett
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In 1978 I worked for Great Northern Paper Company in Northern Maine during the summer months doing water quality studies on the Penobscot River. We had one of these 15 Johnsons at the time and the mechanics hated it. I was the only member of the team that could make it run.

45 years later I did a favor for my BIL and worked on one of these for a friend of his. I spent a month undoing a bunch of stuff and chasing my tail. In my case it was a bad fuel pump, and the Amazon replacement the owner put on was just good enough to run in the barrel. A new OEM pump finally fixed it.

Needless to say, these are one of my least favorite OMC products. Good Luck!

David Bartlett
Pine Tree Boating Club Chapter

"I don't fully understand everything I know!"
August 30, 2025 at 9:02 am #299428
crosbyman
Participant
Canada Member
see diag

Johnson Cylinder & Crankcase Parts for 1975 15hp 15R75C Outboard Motor

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August 30, 2025 at 12:04 pm #299438
Buccaneer
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US Member
I looked at the diagram earlier, and looked at the motor again.
Is the thermostat under the water jacket cover on the head?

I see nothing obvious otherwise. I don’t think there’s even
room to pull the water jacket cover off without hitting
the lower cowling.

Prepare to be boarded!
August 30, 2025 at 1:19 pm #299450
crosbyman
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Canada Member
see part#19 ! should be easy to find..

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August 30, 2025 at 2:05 pm #299454
Buccaneer
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US Member
Crosby Man, I seen the thermostat on the diagram, but
not on the motor.
Photos below.
Please circle the thermostat and send it back, lol.

I just got back from the lake with the motor, and it ran great.
It really scooted my little Alumacraft along!
I leaned out the low speed needle quite a bit until I got
a little hesitation opening it up, then went back a little richer.

Prepare to be boarded!
August 30, 2025 at 2:59 pm #299461
crosbyman
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Canada Member
well I don’t see it but what is that sylver circular bulge just left of the lower side bolt of the cyl head…. is there any chance the thing could be behind a cyl head cover necessitating its removeval ??

if not ….I guess the info on marineengine.com is wrong or your PH is

cyl head pic on marineengine.com show a cavity to fit the thermostat on the INSIDE of the cyl. head…. then a cover fitting over it ??

Evinrude Johnson OMC 0320495 – Cylinder Head (320495)

see covering section over the cyl head thermostat cavity

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August 30, 2025 at 4:47 pm #299469
Steve D
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It’s the bulge…
August 30, 2025 at 6:09 pm #299473
Buccaneer
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US Member
I’m guessing the thermostat is behind the water jacket cover.
As I mentioned earlier, if that’s the case, it doesn’t look
like it’s possible to pull the cover with the lower cowl
in the way.
If I get to talk to the owner, I’ll ask him if he knows if there’s
a thermostat in place.

Thanks.

Prepare to be boarded!
August 31, 2025 at 8:30 am #299476
Steve D
Participant
I’ve only worked on a couple motors from that era Buc, and never had to try and remove the water jacket to check or replace the thermostat. Is it possible that maybe once you move the hose out of the way, and remove the spark plug and wire, there might be just enough room to get a box wrench on that lower screw and possibly enough clearance for it to be fully backed out? I’m sure someone with more experience with those (maybe fleetwin?) will shed some light on it.
August 31, 2025 at 8:49 am #299477
labrador-guy
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US MEMBER PAY BY CHECK
Buccaneer wrote:

fleetwin wrote:

These things definitely have thermostats, and I would absolutely replace it if it is missing. Won’t idle worth a darn without warming up and will foul plugs.

Be careful, there are two different styles of heads you might find on this thing, each has its own unique gasket. Post a picture of the head and I will get you the gasket part number. These thermostats are a pain to replace, but it can be done, am guessing this has not been used in salt water.

I watched the video, seemed to run OK, your mixture adjustment seems right.

Fleetwin, I guess I better look at the parts diagram, as I didn’t
see anything atop the head or block that looked to be hiding
a thermostat. Thanks!

P.S.- What do you know about those plastic gas tank cap’s
venting or pressure relief systems?

Buc on my ’74, the thermostat was at the bottom of the head. Not the best thinking in my opinion. My motor is gone now and I can say good reddens!

dale
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