Differences between a ’73 and ’81 25 HP Rude??

[instigator]

I’ve got a ’73 20 hp that I put a 25 hp carburetor on that runs perfect on my inflatable.

I just stumbled on a ’81 25 hp w/good compression but ugly.
Trying to see if theres anything on the ’81 making it worth swapping w/the ’73??

Powerheads appear very similar but the ’81 has angled plugs where the ’73’s are straight.
Both have compression in the 150 range.

The ’81’s power is rated at 5000 RPM and I think the ’73 is only at 4500 but is rubbed off the tag.

I checked the BRP parts site and see the blocks, heads and intakes are different but thats all I know.

Anyone know these better than me??

Thanks guys.

Gary

[frankr]

The ’81 is a very different motor. More cubic inches. Basically, it is the 35hp detuned to make 25 horses. And they slapped the old 22 cubic inch motor’s lower unit on it. I suppose you could look at it as a hybred. Later years got the thru-prop exhaust used on the 35hp.

[instigator]

Thank you Frank!

Exactly what I was hoping to hear.
A friend told me this motor is a 35 w/a carb change but cant believe thats the only thing different.
??

[garry-in-michigan]

Instigator stated;

quote :

I’d rather be competitive w/junk I built in the garage than win w/stuff I bought.

Me to . . . 😀

[jerry-ahrens]

That’s a neat picture Gary. Who was it in the picture ?

[jerry-ahrens]

I owned a 1981 25 Evinrude for over 20 years, and finally sold it with a jon boat that I wasn’t using anymore. That was the quickest 25 that I’ve ever ran. I still have a 1978 35 Evinrude that runs good but is not near as fast as the 25, even though it had a gazzilion hours on it. Several 14 ft. V bottom aluminum hulls that I had that 25 on, were simply not enough boat to handle the power. I kinda hated to let that motor go, but with over 100 outboards I finally did.

[PugetSoundBoater]

I looked up both years on Marine Engine.com. Without knowing the exact models, it does appear that the 1973 has points and condensers , the 1981 has CD ignition. If it would fit you could swap the ’81 CD ignition system to your 1973 and be better off. As for an exact fit, model numbers would help. Someone here should have the definite answers to this swap.
I have a ’81 Johnson 25 ,converting to a long shaft and i can’t wait to get it out on the water this spring .

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[PugetSoundBoater]

Now i see the 73 is a 20HP ,the ’81 is a 25hp. May have some differences for an ignition swap. Someone will have a real final answer here.

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[dan-in-tn]

Instigator; Just so you are not confused (or maybe I am). Not much of anything off the ’81 motor will fit the ’73 motor. Some pieces look similar, but are not the same. As FrankR mentions these are two different family of engines. Without going into every detail (you can compare part numbers on ME) the powerhead won’t fit your exhaust housing. Not sure your lower pans & motor cover would even fit over the larger powerhead? The earlier powerhead was before the interim ignition system used before CD so not sure you could adapt the later ignition to the early motor. Anything can be done, but it is not a bolt on. The late gearcase has a different driveshaft than the early one. I think the water pump is even different. The late gearcase is a three lug shifter which would be good, but again not sure your old driveshaft can be made work with early driveshaft? The ’81 has a TR style carburetor which is a larger mounting flange. About nothing is the same.
As far as power output. The 32 cu in motors didn’t become 35hp (30hp after prop rated) until they got the thru hub exhaust. That requires a totally re-designed midsection. The old split line gearcase never put out that type horsepower. Even though they marketed a 28SPL with that old style gearcase.
Just wanted to make sure you understand what you have. Hope that helps.

Dan in TN

[fleetwin]

Like everyone has said, virtually nothing is swappable between these two engines. The older 20hp runs OK as it is, correct? If so, please leave it alone and enjoy it the way it is.
The 81 25hp is a nice engine and worth some work to get it up and running, even though it is "ugly", this way you will have two running engines. Post some pictures of this engine once you start looking at it more closely.

[Author]

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