Johnson 25E74M

[spring]

I'd like to start a thread for this engine. I have lots of questions about it. I will take any advice I can get. If some of you already have experience with the engine, and how to set it up for a boat like the Wetback, please sing out.

The boat finished, less engine, will weight around 150 lbs. Some say they are as light as 115 lbs, others say 175 lbs. I am curious about propellor choices. They appear to be limited. As well, I'll be asking about the lower end and can the gear ratio be changed, or the lower end swapped out for something with a lower ratio.

They will all be newbie questions, but that's where I'm at.


Cheers!

[58 Johnson]

I hate being the downer when it comes to old OMC motors (or is it perhaps the voice of reason?)...

ugh... my message of several paragraphs is gone.  I should have been saving it elsewhere as I went along.  Will repost when I stop swearing!!   

[58 Johnson]

Part II !!!

The powerhead in your Johnson 25 was first developed as an 18 hp for the 1957 model year. The same powerhead was used from that time right through until the late 70s, maybe 1980, as well. With various carburetor configurations sometimes it was an 18, sometimes it was a 20 and for a few years it was even a 25 horse power. I have a 1972 Evinrude 25 myself.

It was not particularly fast. And it only came with one choice of propeller; the one that came with it. They are easy to start, easy to maintain, and easy to fix motors. They are super dependable and long lasting. But speed was not among its virtues.

I’m not aware that there’s anything like a high-speed lower unit for it. Having said that, I have seen an altered exhaust housing and high speed lower unit built for an old Johnson 35. So who knows what might be out there?

The propeller might also be a challenge. I suspect yours, being a 1974, was a shear pin drive. I think it was around 1976 when OMC went to a spline drive. But the pin drive may be a blessing because there tended to be a larger variety of props for older motors. The two best sources I found when working on a previous motor were eBay and Michigan Propeller.  Be patient when you’re on the Michigan Propeller site.  It takes a while to sort through but you may find listings for a higher pitch prop that would fit your motor. But at the same time it’s highly likely they will no longer stock it. But given the part number, the diameter and pitch, you should be able to track it down on eBay over time.

However, it’s only fair that I give you my one small experience with changing the prop on a Johnson 20. It was from the 1960s and I was running on a similarly aged “tinnie”, that is, an aluminum fishing boat. It was dependable an adequate for the job. But I planned to put the 20 on it sea flea that I had rebuilt. I asked my local prop shop to increase the pitch on the standard old aluminum prop.

I got exactly 2 more miles per hour from the 1”  increase in pitch. And I was pleased. However the downside was that it became gutless as a utility motor with any kind of load in the aluminum boat. Somehow the 2 mph increase wasn’t so exciting when I figured in the loss I was taking as well.  The powerhead and the drive system and the propeller seem very well matched when they come as the original set up.

These motors were never built to be particularly fast. They were built to run forever.

Anyway, I hope that doesn’t rain on your parade. As I mentioned I have a 25 of my own of the same vintage. If anyone can find a way to make that thing go faster, I would love to hear about it. I can go about 27 mph in my 250 pound 11 foot “runabout” (not quite a sea flea). Good luck with your adventure.

[58 Johnson]

P.S.

You always have the option of having a propeller made as well. There is some expertise on the site and you probably wouldn’t have any trouble finding a prop builder. 

Several years back we had a regular contributor to the site from southeast U.S. who had his old Evinrude family runabout motor pushing 50 miles an hour on a speed boat. He had specially made props. But he had an advantage, because his friend was building the props for him in a local shop!

[spring]

Thanks Ross.

This boat's racing days are over. So are mine. That said, I'm hoping to get it going fast enough to scare myself.

With this engine, by asking these questions it may appear that I am getting ahead of myself. Really what I am trying to do is to figure out how much potential the engine has.

Thanks for your input.

Cheers!

[spring]

I'm starting to wrap my head around getting the motor ready. Buddy I bought it from said it ran great. Hold that thought!

First up was to check for both compression and spark. Compression seems to be around 90, but the engine hasn't run in 4 years. 90 will get me started. It has spark so I'm going to leave it at that. I won't pull the flywheel to inspect.

The next task was to remove the starter, choke solenoid, and associated wiring. The engine components alone cost about 10 lbs. I can do without them. As well, with this engine being an E, it had pigtails for the shutoff. I can use those to wire in a kill switch.

I pulled the carb and gave it the once over with a new kit. Buddy said it ran well? There was absolutely no way it ran at all. The float valve was gummed up solid. There is no way it happened overnight or even from sitting for the time it did. There was years of accumulation in there. The wire spring that holds the float valve to the float was gone. Once I removed the float I had to use copious amounts of brake cleaner to loosen some of the grunge. Even at that I had to carefully remove the needle valve with a pair of pliers. Everything is cleaned up now and back together.

I have a seal kit for the bottom end and an impellor. I'll get to that shortly. When I was inspecting the motor during purchase it appeared the lower end was dry. When I got it home I checked it. There was oil, but predictably there was also water. I'm dreading opening it up.

I picked up both a Tiny Tach and a Michigan Wheel 11" prop. That will get me started. I also have a brass 2 blader with about a 14" pitch. I bought it for scrap value. If I need more pitch I might McGyver the brass prop to fit.

I am in the process of building a throttle. I'm too cheap to buy one. I've mocked up a few ideas. I'm using the E brake cable from a Ford Focus. I have hand made fittings at both ends of the cable to mate with the throttle lever and the engine. I think I have something workable. I'll need the motor on the transom to be sure. If it works out according to plan it should look kind of cool.

And that's about it for now. I hope to run the engine within the next couple of weeks.

Cheers!

[spring]

From the day I purchased the motor, I worried about what I'd find when I opened up the bottom end. I was expecting a bunch of rusty bits. Instead I was pleasantly surprised. Although, the case had leaked water in and oil out, things seemed to be in pretty good shape.

I had bought a seal kit to install. I kind of hit a wall when it came to removing the driveshaft seal. Rather than risking damaging the bearing I deferred to experts. Fortunately for me there is a guy a couple of blocks away that said he'd do the remove and replace for me. I dropped everything he needs off to him yesterday and expect to hear back shortly.

Also fortunately, this engine is an E model which has made hooking up a kill switch easy. The carb has been overhauled and is back on, so once I get the bottom end back together and hung back on I should be ready for a tub run.

I'm going to start with an 11" prop. I suspect it will not be enough but it will get me going. Depending on how everything goes I can plan next steps after the first runs.

BTW the checker plate on the transom is actually a jack plate. I have lots of options when it comes to trimming.

Cheers!

[58 Johnson]

You’re cookin’ now!  Good work on the lower unit.

I was never able to find a prop with a higher pitch than 12” for this motor (and all associated horsepowers before and after your model year).  If you find one please let us know.

Also, I refer you back to an earlier comment.  Be careful how far you jump between pitches, assuming you have that choice and money for multiple propellers.  In my experience, this is a mild mannered, low revving motor whose power range may have little tolerance or ability to go to a pitch that is much higher than stock. 

Good luck!  I’m all ears! 

[spring]

The Johnson has been tub run and is ready to go. My problem now is a suitable trailer. Everything is stalled until I come up with something. I borrowed one from a buddy, but it doesn't lend itself to being easily modified to fit Billy G.

I have also been pretty busy with other projects. Hopefully I'll get a chance to sort this next week.

Cheers!

[58 Johnson]

The weather in southern Ontario has been near perfect lately and will be again next week.  I hope you’re able to take advantage of it. 

Great picture of the (near) complete boat and motor set up.  You’ve taken us with you on the rebuilding journey.  Thank you and congratulations!  And good luck with the test run!

[spring]

As you may have read elsewhere, the Wetback is in the water. The engine starts first pull every time and runs smoothly. It is perhaps under pitched with an 11' on it but it will do for now. Speed, predictably is not stellar.

After the initial test runs I made one minor pitch adjustment at the transom. I have yet to test it.

During the runs the boat would accelerate to a point where it appeared it had a slight tendency to plough. Shifting my weight couldn't help it. I'm figuring a slight change in thrust line should do it. If so, there will be more speed still to be found. The good news is that as was there was no tendency to porpoise.

The instructions for the tiny tach advise not to change the length of the leads. Information on the internet says changing length is no problem. I had no choice. I had to both lengthen the leads and put a quick disconnect in them. The result is a tach that reads incorrectly. That is not a huge deal. I'll putter with that later. In the meantime I'll guess rpm by ear.

I was concerned about cooling. I added a clear plastic tube to the water outlet that routes forward to where I can see the end of it. If water is going through the tube I obviously have cooling. That works well at low speed, not so well at high speed which is where I really need it. For the test runs I used an observer in a chase boat to confirm the engine was getting cooling water.

The water cooling thing is very important in that the engine is set such that once on the step the exhaust sound becomes loud. The reason for that is that the exhaust outlet which would normally be below water level is now exposed. The water intake is on the forward side of the exhaust outlet. While the exhaust outlet may be clear of the water, it is vital that the water intake is not. So far I have used two cues to ensure the engine is getting water. First, the observer in the chase boat. Second, the fact that the propellor is not cavitating. The water intake is at a slightly lower level than the top of the propellor blade. I'm figuring if the blade is in the water, the intake also must be. However, I'd like to get the water tube indicator working better for peace of mind.

And that's where its at. I bought the Johnson at a pig at a poke, and a price within my snack bracket. I think I got a fair deal.

Cheers!

[spring]

A bit of an update if anyone is interested.

The Tiny tach now works reliably. It sure is handy. Due perhaps to hysterisis (play) in the throttle linkage, setting the idle is difficult. I can set it to 750 rpm in neutral as per the book but that doesn't gurantee that if I advance the throttle and return it to the idle stop I'll get the same rpm. Guaranteed I won't if the engine is in gear. Most times the engine shuts down with the throttle on the stop. That, for me, is not really a problem. It is more of a benefit.

To start the engine, with it in neutral,  I visually set the throttle linkage to the start position. One pull on the cord and away it goes. I can then adjust the throttle as required to give 750 rpm -0 +50. That reference is a good one to have when shifting out of neutral to prevent the engine stalling. (Remember I removed the tiller.) All other engine operation is normal. When I choose to idle, as in when approaching a dock, since the engine is in gear, I'll use the Tiny tach as a reference to set the engine rpm to 750 -0 +50. When I am finished with the engine, rather than use the kill switch, or reach around to manually select neutral, I just  snap the throttle back to its lowest position and the engine quits. Doing it this way is actually quite handy, not a lot different than the throttle on a jet engine.

I now have the boat set up so that wide open, on the step the Tiny tach reads 5300 rpm. I'm happy with that. If I'm not getting 5300, I know that there is more that I can do such as shifting my weight slightly forward. 5300 rpm corresponds with the sweet spot on the step. Too easy. A lighter pilot will no doubt yield a higher rpm, but still within the safe engine rpm range.

4500 rpm appears to be another key. If I get much lower than that, the engine takes time to accelerate. If I can maintain at least 4500 rpm I seem to be in the torque range where it accelerates well. So manoeuvering through wakes etc often becomes a matter of visually seing what needs to be done, and planning how to do it at 4500 rpm or above. If I am on the step but have had to slow down and am about to turn, as I begin to turn I'll add throttle to maintain rpm and thereby stay on the step.

I am now satisfied that the water tube cooling indicator is likely not required for reasons I mentioned in the previous post, but I think I'll keep it.

I am very pleased with what a smooth running engine this is.

Cheers!

[58 Johnson]

I love the detail and the dedication.  I would bet that no one has ever so seriously diagnosed and catalogued the details of a plain old Johnson 25!  Pretty impressive!  And thank you!