Author Topic: Johnson 25E74M  (Read 61 times)

spring

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Johnson 25E74M
« on: January 26, 2020, 07:37:04 PM »
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!
« Last Edit: January 26, 2020, 11:25:33 PM by spring »

58 Johnson

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Re: Johnson 25E74M
« Reply #1 on: January 27, 2020, 01:22:53 AM »
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!!   :| :wink: :-D
« Last Edit: January 27, 2020, 01:25:15 AM by 58 Johnson »
Ross Jewiss, Huntsville, Ontario

58 Johnson

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Re: Johnson 25E74M
« Reply #2 on: January 27, 2020, 01:54:01 AM »
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.

Ross Jewiss, Huntsville, Ontario

58 Johnson

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Re: Johnson 25E74M
« Reply #3 on: January 27, 2020, 01:58:36 AM »
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!
Ross Jewiss, Huntsville, Ontario

spring

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Re: Johnson 25E74M
« Reply #4 on: January 27, 2020, 02:56:52 AM »
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!