New to me 1977 Johnson 25hp Questions

[shane-r13]

Hi Everyone,

I’ve recently obtained a new to me 1977 Johnson 25hp.

I have just basic questions on this motor.

1. Oil mix – I assume 50:1?

2. tell-tale location? i have a little dribble out of a tube on lower cowling but most water comes out the exhaust. Is this correct?

3. Shear-Pin? I had a 1976 15hp but was slip clutch and i think this one is shear-pin. Seems odd to step back in tech a year later.

4. Thinking of maybe converting to electric start, already have the correct flywheel, I need the starter bracket, quick search on eBay makes them seem hard to find. Is it a rare to have electric start on these motors and what year and model motors will fit this for electric start parts?

5. Correct spark plug?

I’m 100% sure I’ll be adding more questions as i dig into this motor over time.

Any help or answers is always appreciated!

Shane

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Attachments:

[fleetwin]

50:1 fuel/oil mix
The overboard indicator hose outlet is located on the starboard side of the lower motor pan. There is a little plastic retainer/restrictor that is located in the end of the hose, debris loves to get caught in it…There should be a fairly strong but narrow stream of water shooting out of this tube. If not, try blowing back through the restrictor, or removing it from the hose and cleaning it out….Usually this is all that is wrong here….
Yes, there will be a fine spray of water/exhaust coming from the two exhaust relief holes as well….
This engine does use a shear pin, later models used the splined shaft like on your 15hp. This is an old gearcase design that has been around for many years.
Electric start on these engines was fairly common, so keep searching for the bracket, you will find it…I’m pretty sure the same bracket was used on the 25-35hp models from 1977 on up through the 90s, but don’t hold me to this statement…Check actual parts lists/numbers to be sure.
The correct spark plug is the Champion L77JC4. The QL77JC4 can be used as well. The original number was L77J4 from Champion…Gapped at:
.030"
Looks like your engine was manufactured in Canada. These are fairly rugged engines, but don’t idle as nicely as models from the 50-60s. The CD ignition is fairly simple and easy to understand/repair. The cooling system can be difficult to service, and watch out for worn clutch dog/gear lugs, replacement parts are very expensive.

[outbdnut2]

I’ve seen that tell-tale hole get clogged by some kind of bug that nested or hibernated there.
Dave

[shane-r13]

Thanks for the info guys.

Is it possible to put a lower end from another model that has the splined shaft instead of shear pin. I generally use my motors on a river that the water level fluctuates drastically. Sometimes its over the banks and sometimes there is 12 inches of water. Hitting logs and rocks are a given and shear pins don’t generally like those.

Well at least the previous owner was using the correct plug!

I’ll have to look at my tell-tale for clogs, as it is currently only has a dribble. I haven’t ran it for very long as I wasn’t sure how the cooling system worked, but it didn’t seem to be overheating. I’ll replace the impeller regardless this winter anyways.

Shane

[frankr]

The 1977 25hp was sort of a lower priced mixture between the "old" 25 and "new" 35hp, and yes it still had the 307949 Stainless steel Drive Pin. 1978 25hp still had the old style lower unit, but with a splined prop shaft. I’ll stick my neck out and say you could probably change the prop shaft (and prop) to a splined version, but I’ve never tried it.

[shane-r13]

quote FrankR:

The 1977 25hp was sort of a lower priced mixture between the “old” 25 and “new” 35hp, and yes it still had the 307949 Stainless steel Drive Pin. 1978 25hp still had the old style lower unit, but with a splined prop shaft. I’ll stick my neck out and say you could probably change the prop shaft (and prop) to a splined version, but I’ve never tried it.

Never thought of just the shaft and prop. I was thinking the whole foot would have to be changed. I’ll have to keep my eye out for those parts. I’m not
in a major rush to modify the motor. It works as it was intended. Although it is not fun running some rapids backwards in a boat trying to change the shear pin. Haha.

[fleetwin]

I just don’t think it is worth all the effort and expense of replacing that prop shaft to take advantage of the splined set up….I would just pack a bunch of shear pins and some cotter pins. The gearcase on the 1977 25hp is pretty unique, special spine set up into the powerhead…Don’t think newer models will bolt up properly…

[shane-r13]

quote fleetwin:

I just don’t think it is worth all the effort and expense of replacing that prop shaft to take advantage of the splined set up….I would just pack a bunch of shear pins and some cotter pins. The gearcase on the 1977 25hp is pretty unique, special spine set up into the powerhead…Don’t think newer models will bolt up properly…

Your likely right, probably cost and take more effort than its worth.

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