1975 25 HP Johnson Conversion to 35 HP?

Acton

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So I was curious to see if a 1975 25 HP Johnson could be converted to a 35 HP by changing out the intake manifold and carb since that can be done with later models in the 80s. The 1975 series doesn't have a 35 HP model, but the 1976 series DOES have a 35 HP model. I've seen topics here that the conversion would work with the later 70s models too, specifically 1976 and later when they switched from the points and magneto ignition to the CDI ignition system.

I took a look at these 2 schematics for the 1975 intake manifold and the 1976 intake manifold and they are exactly identical.

1975 25 HP Intake Manifold Schematic
See attachment below

1976 25 HP Intake Manifold Schematic
See attachment below

Then I took a look at the 1976 35 HP Intake manifold schematic, it seems like there other parts other then the intake manifold that I might have to replace, including the leaf plate and intake manifold gasket, they are of a different design then the 1976 25 HP model. In the topics I link too below it seems to be the consensus that all that would need to be replaced to do this conversion would be to replace the intake manifold itself and the carb from the 35 HP model. Can I just leave the leaf plate and gasket from the 25 HP model, or do I need to buy the 35 HP leaf plate and gasket IN ADDITION to the carb and intake manifold? Does the different leaf plate design not effect this conversion?

1976 35 HP Intake Manifold Schematic
See attachment below

Many topics that cover the conversion process mention that the only thing that's needed is to throw on a the 35 HP Carb and Intake manifold for this to work. Is it necessary in my case to buy additional parts, other then just the 35 HP intake manifold and the 35 HP carb to do the conversion on a 1975? Do I need to buy the 1976 35 HP gasket and leaf plate in addition to the intake manifold, or can I just use the gasket and leaf plate that's currently on my 1975 25 HP Johnson? Also what model years use the 0386792 35 HP Intake Manifold and the 0387271 35 HP carb? Do both Johnson and Evinrude 35 HP motors use the same exact parts?

For reference I'm attaching a link to two different articles that cover this topic more in depth

80's Johnson/Evinrude 25hp to 30-35hp conversion with pics
http://forums.iboats.com/forum/engin...sion-with-pics

Tinboats Johnson/Evinrude 25hp to 30-35hp conversion with pics
http://www.tinboats.net/forum/viewto...0c374e96048cfd
 
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Acton

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Attachments below for 1975 25 HP Intake Manifold (1), 1976 25 HP Intake Manifold (2), 1976 35 HP intake Manifold (3)
 

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jimmbo

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Not Possible. The 75 25 hp is a 22 cubic inch block, the 35 which was introduced in 1976 is a 31.8 cubic inch block. The 25 hp of that era was a souped up version of the 18/20 hp engine of the time
In 1977 a new 25 hp was introduced that had the same displacement as the 35hp, but even there you could not turn a 25 into a 35. the blocks and cylinder heads were different, as well as carb, reeds, exhaust manifold, midsection, lower unit, etc. Around 1985, the engines shared more parts, but a complete comparison of the parts lists to find the differences is required. Genuine dealer parts lists, as I've found some online lists to have major typos.
 
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F_R

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Agreed, your 22 cubic inch 25hp is already a souped-up 18. You just aren't going to get any more out of it. And no, you can't slap a 31.8 CU powerhead on it either.
 

Scott Danforth

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to convert your motor from 25hp to 35, there are two intermediate steps. sell the 25 converting it to cash. then take the cash and convert it to a 35hp
 

jimmbo

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to convert your motor from 25hp to 35, there are two intermediate steps. sell the 25 converting it to cash. then take the cash and convert it to a 35hp

That is the easiest and most reliable way to get more horsepower
 

Acton

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Not Possible. The 75 25 hp is a 22 cubic inch block, the 35 which was introduced in 1976 is a 31.8 cubic inch block. The 25 hp of that era was a souped up version of the 18/20 hp engine of the time
In 1977 a new 25 hp was introduced that had the same displacement as the 35hp, but even there you could not turn a 25 into a 35. the blocks and cylinder heads were different, as well as carb, reeds, exhaust manifold, midsection, lower unit, etc. Around 1985, the engines shared more parts, but a complete comparison of the parts lists to find the differences is required. Genuine dealer parts lists, as I've found some online lists to have major typos.

Alright so I'd have to check the parts list to know what years the conversion works on exactly.

From the first topic I linked to here on iboats they mention later on on the 4th page that the conversion would work even though the lower unit on the 25 had a split gear case which was different than the 35. It seems like you guys knew right away that it wouldnt work on the 75, does anyone have an idea of what years the conversion is possible for, I know there might be some differences in the lower unit, but from what I understand about the series of engines that the conversion is possible for, it's because the power heads are identical. Much like on the 9.9 and 15hp series of that era. When I do look at part numbers, I'll be looking to see that the power heads are identical to know that the conversion will work, correct?

Anyway my question was answered regarding the 75 model, thanks.

Edit: Found my answer in this topic:

http://forums.iboats.com/forum/engi...ards/187596-25hp-johnson-the-good-and-the-bad

Looks like the 31.8 c.i. power heads on the 25s started in 77.
 
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jimmbo

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When I said the engines from 1985 sharing more parts... That is they shared parts only amongst themselves, not with the earlier engines. Parts that may be different between the various HPs of the 31.8 cu.in. engines. Blocks(different porting), cylinder heads(chamber size. affects compression.), reeds, reed stops, intake manifolds, carbs, exhaust manifolds, exhaust tuners, throttle cams(affects sync between carb and timing). Until 1985, only the 35 had the one piece gearcase with thru hub exhaust and bigger transom bracket. The lower HP versions had the split case with separate exhaust, with a more basic transom bracket. So if one was able to put the 35 powerhead on the 25 midsection, you still wouldn't have a 35.
I do know that the 1976 - 1980 35hp and the 1977 - 1980 25hp shared very few parts, a few bolts, clamp screws, starter recoils, and cowls would swap. We owned both the 25 and the 35 from those years.
You really would be better just finding a used 35/30 hp in good shape. It will cost less than trying to find the proper parts needed.
 

F_R

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When the 25hp used the 31.8 CU powerhead and the split gearcase, it also had a different midsection to join it all together. And that isn't all---might as well accept the fact that they are different motors.
 

racerone

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I think I am going to develop an adapter kit to put a set of 60 hp carburetors on the 31.8 cubic inch block.-----Should be a popular item / big seller.
 

Scott Danforth

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I think I am going to develop an adapter kit to put a set of 60 hp carburetors on the 31.8 cubic inch block.-----Should be a popular item / big seller.

Why stop there. tuned exhaust and a small 40mm turbo to go with it
 

Acton

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When I said the engines from 1985 sharing more parts... That is they shared parts only amongst themselves, not with the earlier engines. Parts that may be different between the various HPs of the 31.8 cu.in. engines. Blocks(different porting), cylinder heads(chamber size. affects compression.), reeds, reed stops, intake manifolds, carbs, exhaust manifolds, exhaust tuners, throttle cams(affects sync between carb and timing). Until 1985, only the 35 had the one piece gearcase with thru hub exhaust and bigger transom bracket. The lower HP versions had the split case with separate exhaust, with a more basic transom bracket. So if one was able to put the 35 powerhead on the 25 midsection, you still wouldn't have a 35.
I do know that the 1976 - 1980 35hp and the 1977 - 1980 25hp shared very few parts, a few bolts, clamp screws, starter recoils, and cowls would swap. We owned both the 25 and the 35 from those years.
You really would be better just finding a used 35/30 hp in good shape. It will cost less than trying to find the proper parts needed.


I agree, it would be ideal to just go out and get the 35 HP in the first place, that being said we live in the real world and who knows what kind of deal I could get on a 25 HP motor in the future. Its good to know what series of motors this "conversion" works on.

There was discussion in the topics I linked too about other differences between the 25 HP and 35 HP motors, but at the end of the day people who did the conversion saw a pretty big increase in speed at WOT. I saw that your point was brought up about the split gearcase and basic transom bracket on the 25 HP motor in those topics as well, and I understand that anyone doing this conversion should know that and go about the conversion at their own risk. A lot of people felt that the lower unit would be able to handle it, and went ahead and did the conversion anyway. If I owned a 25 HP motor, I'd do the conversion myself no questions asked.

Here is such a quote from the topic I linked too above. Post #67
http://forums.iboats.com/forum/engin...ith-pics/page5

But the difference betweent the 25 and 35 in 78' was carburetor, intake, gearcase (25 had smaller split case and the 35 had the larger solid case) and the exhaust housing assembly had different parts on it. The exhaust tube was identical though. The 35hp in 78' had a larger more beefy transom mount with different little parts on it but none of it was for performance increase, just a heavier setup. I have seen people still do this mod with the smaller gearcase and have had no issues. Also I run the smaller transom bracket assembly on my 88' and have no issues either.

At the end of the day this conversion cost just about $100, seems like it would be well worth it to me. For someone like me who doesn't own the 38.1 cu. in. 25 HP motor, it would just be smarter to go out and buy the 35 HP motor in the first place.
 

jimmbo

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You have a 22 cu in motor, you have been told several times it is next to IMPOSSIBLE to get any more out of it and still be happy with it.

The 35 had a higher(numerical) gear ratio than any of the pre1985 25s and was able to use bigger props with blades that had higher rake, more pitch, and a more efficient design. The cylinder heads were different, due to differences in compression and in the cooling system. As I said earlier, we had a 79 25hp and an 80 35hp. There were many subtle differences.

More chances were made to the 35's exhaust in 1981, it was said to be a tuned exhaust. Exhaust tube was still the same but changes were made to the block and inside of the exhaust manifold. It took me a long time to find the differences, and they could not be retro fitted to the 1980, unless you changed the block and internal manifold. However when you look at the Service Parts list the one number replaces both blocks, leading a lot of people to think the parts are the same. The were not the same from the factory, but the Service(replacement) part is the same, but will require some additional parts to complete the job. So when comparing Parts on Engines, Service parts lists will not always give an accurate picture as to what differences the engine had, You need access to the OEM parts lists.
 

Acton

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You have a 22 cu in motor, you have been told several times it is next to IMPOSSIBLE to get any more out of it and still be happy with it.

​I understand that.

At the end of the day this conversion cost just about $100, seems like it would be well worth it to me. For someone like me who doesn't own the 38.1 cu. in. 25 HP motor, it would just be smarter to go out and buy the 35 HP motor in the first place.

Anyway my question was answered regarding the 75 model, thanks.

More chances were made to the 35's exhaust in 1981, it was said to be a tuned exhaust. Exhaust tube was still the same but changes were made to the block and inside of the exhaust manifold. It took me a long time to find the differences, and they could not be retro fitted to the 1980, unless you changed the block and internal manifold. However when you look at the Service Parts list the one number replaces both blocks, leading a lot of people to think the parts are the same. The were not the same from the factory, but the Service(replacement) part is the same, but will require some additional parts to complete the job. So when comparing Parts on Engines, Service parts lists will not always give an accurate picture as to what differences the engine had, You need access to the OEM parts lists.

Alright, fair enough. I'm no expert in outboards so I'll take your word for it. Seems like you've got actual hands on experience with these motors, I'm just some guy on the internet reading up on what others have posted.
 

Scott Danforth

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I'm just some guy on the internet reading up on what others have posted.

Everything on the internet is true.....they cant lie on the internet..... Im a french model

https://www.youtube.com/watch?v=bufTna0WArc

Bon Jour


seriously though. these guys know what their talking about. your vintage 25hp is at the top of its power output. get a different series of motors and the 25 is at the bottom of the power output.

people claim big power gains from putting large carbs on small motors. in truth, they muffed up the motors ability to run at anything other than WOT and say that just to convince themselves.
 
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Acton

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Everything on the internet is true.....they cant lie on the internet..... Im a french model

https://www.youtube.com/watch?v=bufTna0WArc

Bon Jour


seriously though. these guys know what their talking about. your vintage 25hp is at the top of its power output. get a different series of motors and the 25 is at the bottom of the power output.

people claim big power gains from putting large carbs on small motors. in truth, they muffed up the motors ability to run at anything other than WOT and say that just to convince themselves.

I'm not debating that the conversion will or wont work on my 75. Being that the block was a different size completely then the 25s that came later on in the 80s is enough for me to understand it won't work on my motor.

I remember when that ad came out on TV, too funny! Definitely gotta take what everyone says on the internet says with a grain of salt. That being said I really appreciate everyone's input in this topic. I definitely came away with more knowledge then I had when I started the topic. My thanks goes out to everyone who contributed, I know a lot of you guys are regulars on here and I for one really appreciate it that you guys always seem willing to give advice.
 
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