1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

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Johny25
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Location: North Idaho

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by Johny25 »

Ok so anyone who owns a 80's Johnson 20/25 has heard of being able to up the HP to a 30/35 well it can be done and I did it with very good results. I also believe this can be done on late 70's and early 90's models but you will have to cross reference the parts differences as I did with mine.

Ok so I bought a 88' 25hp Johnson several weeks ago for $550 bucks (stole it basically lol) and the first thing I did when I got it home was to find out if I could get more hp out of it. I did a mod on my 87' 9.9 so I was pretty sure I could do it to the 25 also. And sure enough I cross referenced the difference between the 25 and 30/35 in 1988 and the only difference was carburetor and intake manifold. Now I say 30/35 because in 1985 the hp rating changed from the crank to the prop which made the 35hp models into 30hps from 85 on. With an exception in 87 when they made a de-tuned 40 into a 35hp. Anyway so I did the mod and took pics of the differences and decided to share with everyone. The mod cost me a total of about $95 bucks if you include gaskets and carb rebuild kit which is mandatory when buying a used carb.

Ok here are the differences in the 25 and 30hp (20hp models usually have even a smaller intake manifold but same carb as the 25)
I bought a 1984 35hp carb and intake off of a military 35hp johnson or MARS edition. Which mounted up fine to my 88, got it off of ebay for $70 bucks with no shipping. (guy lived near me so I picked it up)

First pic is the difference in the front of the carbs. The 35 carb is on the left and 25 carb on the right. The throat of the 35 is about 9/16 inch larger in diameter. (if memory serves me correct?)
DSCN6363.JPG

The second pic is of the back of the carbs which mount to the intake, 35 on the left and 25 on the right. Once again you can see the size difference. The 35 is 1.5 inch diameter and the 25 is 1 inch I believe. Obviously the jets in the 35 are much bigger also. Seem to remember someone saying the 25 carb had a 23mm jet and the 35 had a 32-35mm jet but don't quote me on that.
DSCN6365.JPG
Next is the intake manifolds. Now I have heard people say they ported out their intake instead of buying the 35hp intake. This can be done but it is impossible from what I see to actually get the volume of the 35hp intake from a 25 or 20hp intake. The 35hp intake body is larger than the 20/25 so you will never get the volume of air flow out of a 20/25 by porting it in my opinion.

Here is a pic of the inside, 25 intake on the left and 35 on the right. See the gasket ring on the 25, the outside of that ring is the actual size of the 35hp intake.
DSCN6366.JPG
Here is a pic of the 35 in front and 25 behind it to show the size difference again.
DSCN6369.JPG
And a couple pics showing the difference in width
The 35 at 5 and 5/16
DSCN6370.JPG
And the 25 at 5 and 1/16
DSCN6371.JPG
Here is the intake hole differences.
The 35 at 1.5 inches
DSCN6373.JPG
And the 25 at just over 1 and 3/8 inches
DSCN6375.JPG
And here is the 25 carb mounted on the 25 intake
DSCN6377.JPG
And the 35 carb on the 35 intake is perfectly flush with no flow restriction
DSCN6379.JPG
And the last difference in the intakes was the primer nipple on the bottom of the intakes, the 35hp intake is much larger than the 25 nipple (had to get a larger fuel line hose for this also)
The 35 nipple
DSCN6381.JPG
And the 25 nipple which you can barely see in the pic
DSCN6382.JPG
Now I forgot to take pics of the measurements I made when the intakes were attached to the reed plate but I will tell you this, the 35hp intake when measured through the intake hole to the reed plate is 1/8 of an inch taller than the 25 intake. Meaning that the intake body of the 35 is bigger than the 25 as I mentioned above. So I do not see how you could port the 20 or 25 intake out to match the air flow and volume of the 35 intake.

Ok I never ran the motor on my boat before the mod but I did run it several times in a barrel. And without question the mod made a significant difference in power! Not only could you here and feel the difference but the motor now idles WAY better. And I had done a carb rebuild on the 25 also so I know the 25 carb was fine. It is like the motor was made to have more fuel and air given to it. That is just my opinion and the results I got first hand. Had to do a link and sync to get the advance and carb lined up. I will have her on the water in 2 days and will report how she runs on the boat.

Here is a pic of the motor, it is an electric start long shaft ( I gave her a new paint job and shiny stainless prop also)
DSCN6406.JPG
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Last edited by Johny25 on 13 Apr 2014, 02:14, edited 5 times in total.
BOATS:
2002 Lowe 1467t
1988 Bluefin Sportsman 1900
1989 Spectrum 1900

MOTORS:
69' 6 hp Evinrude
71' 6 hp Johnson
85' 90 hp Evinrude
87' 9.9 hp Johnson (modified to 15hp)
88' 25 hp Johnson (modified to 30hp)
93' 120 hp Johnson

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bigwave
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Joined: 18 Jan 2012, 14:40
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Location: St. Petersburg, Fl

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by bigwave »

Very interesting? Will the increase in hp have any effect on the gear reductions to the prop? How much speed do you think you will gain with the increase in HP? Is there and advantage? Just curious. I might just do this to my boat in the future if there would be a gain in speed vs weight of stuff I am adding to the boat.


Thanks
Kevin
1984 14 Deep Fisherman Mirrorcraft
https://www.tinboats.net/forum/viewtopic.php?t=23315

JMichael
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1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by JMichael »

Nice, I wish I could do something like this to my old merc 20hp. Did you increase the prop dia or pitch for the added power?

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Johny25
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Location: North Idaho

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by Johny25 »

Ok the 20/25/30/35hps all have the exact same gear cases in most all the years I mentioned so there will be no issues concerning the gear case. It is made to handle this extra 5 or so hp I added. I won't know how much speed I gained because I never ran it on my boat prior to the mod but I have seen plenty of threads where guys have stated GPS speed increases of up 8mph and much better planing ability and better speed while boat is loaded down after this mod. The biggest advantage from what I have researched is the motor runs and idles better (which I will attest to already) and speed increase under a load.

And no I haven't increased the prop dia or pitch yet because I haven't run the boat yet. I will do an RPM check with the 10.5 X 11 pitch stainless I bought. If I am to high RPM then I will go up in pitch accordingly. My boat with just my son, me and all our gear is around 850-900lbs total weight so I won't be running light ever. Add the wife and other son plus their gear and I am going to be over 1000lbs easy a lot of times also. So I may not have any issue with over rev? I will update what I find though :)

The reason I made this post is that I read so many threads talking about this mod but nobody had any pics or proof of the differences in the 20/25/30/35hp motors. And some had said just to change the carb and others said just make the intake hole bigger....etc. So I decided to try and clarify and correct misconceptions about this modification to others in the future. It can be done and there is definite gain in HP. This mod would be most valuable on the 20hp since you could gain at least 10hp out of your motor for around $100 bucks
BOATS:
2002 Lowe 1467t
1988 Bluefin Sportsman 1900
1989 Spectrum 1900

MOTORS:
69' 6 hp Evinrude
71' 6 hp Johnson
85' 90 hp Evinrude
87' 9.9 hp Johnson (modified to 15hp)
88' 25 hp Johnson (modified to 30hp)
93' 120 hp Johnson

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bigwave
Posts: 1577
Joined: 18 Jan 2012, 14:40
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Location: St. Petersburg, Fl

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by bigwave »

I really like this thread....I have a newer 25 johnson
and might do this to mine...........If I decide to do this I will do a speed test before and after. I would only do this to offset the weight gain from my mod. I do not care if the speed increases, but I would like the extra hp for planing. Thanks for your post.
1984 14 Deep Fisherman Mirrorcraft
https://www.tinboats.net/forum/viewtopic.php?t=23315

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Johny25
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Joined: 14 Feb 2012, 22:46
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Location: North Idaho

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by Johny25 »

Another thing I forgot to mention was that when I had the 25hp intake on the motor and I took a flashlight and looked through the intake hole with the carburetor off I could see that the intake actually blocked part of the reed plate. This made it more difficult for the reeds to actually open up in my opinion, and caused air flow restrictions.

Yeah Bigwave if you decide to do the mod let me know your make and model and I would be happy to help cross reference to make sure you can do the mod with your year Johnson. I am not entirely sure you can do it to newer Johnsons but it would be interesting to find out if you could :)
BOATS:
2002 Lowe 1467t
1988 Bluefin Sportsman 1900
1989 Spectrum 1900

MOTORS:
69' 6 hp Evinrude
71' 6 hp Johnson
85' 90 hp Evinrude
87' 9.9 hp Johnson (modified to 15hp)
88' 25 hp Johnson (modified to 30hp)
93' 120 hp Johnson

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Johny25
Posts: 1113
Joined: 14 Feb 2012, 22:46
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Location: North Idaho

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by Johny25 »

Update: I got the motor on the lake today and froze my ass off, 34 degrees out. I am sick ass a dog too but had to run it while I had the break in the weather. Ok so the motor idled and ran like a champ in the driveway while in a barrel. I get it in the water and under a good load and she started bogging out when I tried to open her up. And she died about 50yds off the dock. So off came the engine cover and I started diagnosing. So I found out that she wasn't getting enough fuel and in order to get her to open up and run I had to keep priming the fuel to the intake. So I have some work to do in the garage this weekend.

But that being said, if I kept pumping the primer until I got her full throttle she ran great at WOT. And the boat is a freaking missile now! I actually had to second guess myself for adding the extra HP to the boat. With just me and my son we had to be doing 30mph easy and maybe closer to 35 which is really fast in a 14'. And when I got the wife and other son in the boat the motor barely hesitated to get on plane. Had to be doing close to 30 with all of us in it. Was like I didn't even put the extra 230lbs in the boat. (wife is not a small lady) I could plane at half to 3/4 throttle no problem. Other than the fuel issue I had, the motor is a freaking beast! It may have been my imagination but I was having a hard time keeping the boat from listing to the starboard side because of the prop torque at full throttle. And I sit on the port side when running the motor so we usually list to port? We weren't out there long because the wife and kids got cold and I wanted to get back and figure out my fuel issue.

All the mph readings were a guess though and probably felt faster because of the 34 degree air on my face lol. Either way, I have more than enough HP now and probably to much actually. I never thought I would say that either.
BOATS:
2002 Lowe 1467t
1988 Bluefin Sportsman 1900
1989 Spectrum 1900

MOTORS:
69' 6 hp Evinrude
71' 6 hp Johnson
85' 90 hp Evinrude
87' 9.9 hp Johnson (modified to 15hp)
88' 25 hp Johnson (modified to 30hp)
93' 120 hp Johnson

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bigwave
Posts: 1577
Joined: 18 Jan 2012, 14:40
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Location: St. Petersburg, Fl

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by bigwave »

Very cool Johny, Fyi my motor is a 2000 25hp Johnson, do you think it is feasible to do that to my engine? It was too dark last night but the serial number plate is fully in-tact with alot more info...its a two stroke short shaft.

thanks
Kevin
1984 14 Deep Fisherman Mirrorcraft
https://www.tinboats.net/forum/viewtopic.php?t=23315

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jasper60103
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1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by jasper60103 »

Johny25 wrote:... All the mph readings were a guess though and probably felt faster because of the 34 degree air on my face lol. Either way, I have more than enough HP now and probably to much actually. I never thought I would say that either.
Sounds like a beast. Nice =D>

I seem to recall mine was listing port side at one time, and I was sitting starboard side. I've taken my motor off/on many times so it could have been off center.
2018 Tracker Grizzly 1648
2018 30 HP Suzuki DF30A

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Johny25
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Location: North Idaho

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by Johny25 »

Update to my motor not running as well as I wished she had first run. After replacing all my fuel lines I noticed it was drawing fuel and air through the primer line to the intake and spitting fuel threw the line that attaches to the top of the carb while knob is pushed in. See through hoses are awesome :) Anyway I am assuming that the O rings are bad inside the primer assembly so I will be removing and rebuilding the manual primer today. This should correct the sputtering and cutting out issue I had the other day.
Last edited by Johny25 on 04 Mar 2012, 20:47, edited 1 time in total.
BOATS:
2002 Lowe 1467t
1988 Bluefin Sportsman 1900
1989 Spectrum 1900

MOTORS:
69' 6 hp Evinrude
71' 6 hp Johnson
85' 90 hp Evinrude
87' 9.9 hp Johnson (modified to 15hp)
88' 25 hp Johnson (modified to 30hp)
93' 120 hp Johnson

User avatar
jasper60103
Posts: 1285
Joined: 02 Jul 2009, 07:39
12
Location: Twin Cities, MN.

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by jasper60103 »

Hey, Johny. Good tip on the see through fuel lines. The leak was on the smaller line? What size did you use? I have a lean condition on my 30 hp evinrude. That's probably something I should check too.
It's too bad they didn't make a rebuild kit for the primer. Thanks for the update.

Image
2018 Tracker Grizzly 1648
2018 30 HP Suzuki DF30A

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Johny25
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Joined: 14 Feb 2012, 22:46
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Location: North Idaho

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by Johny25 »

Yes that is the exact line I was talking about. I put the same size line back on, or as close as I could find at least. I had to go to the chainsaw shop to get the see through fuel line. Napa and what not did not carry fuel line that small. It is a yellow color but see through. I also dismantled the manual primer today and I think the check valve it not shutting all the way. Which is allowing air bubbles to go through the line. You can rebuild those primers somewhat by replacing the O rings inside and the spring but the check valves are not replaceable so I am going to order the whole primer assembly and replace it I think.
BOATS:
2002 Lowe 1467t
1988 Bluefin Sportsman 1900
1989 Spectrum 1900

MOTORS:
69' 6 hp Evinrude
71' 6 hp Johnson
85' 90 hp Evinrude
87' 9.9 hp Johnson (modified to 15hp)
88' 25 hp Johnson (modified to 30hp)
93' 120 hp Johnson

User avatar
jasper60103
Posts: 1285
Joined: 02 Jul 2009, 07:39
12
Location: Twin Cities, MN.

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by jasper60103 »

OK, makes sense. I will give it a try.
FYI. I've found that boats.net seem to have the best pricing on OEM parts.
Even cheaper than my local OMC dealers with shipping charges.
Thanks.
2018 Tracker Grizzly 1648
2018 30 HP Suzuki DF30A

DIZZYSPOTS
Posts: 40
Joined: 15 Aug 2010, 21:43
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Location: Mescal AZ

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by DIZZYSPOTS »

Thanks for taking the time to post this info...on a parts search now...
also have most of the parts ready for the shifter mod as well

cheers

Mike

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benjineer
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Location: Ellisville, MS

1980's Johnson/Evinrude 20/25hp conversion to 30hp with pics

Post by benjineer »

I've got an '03 I want to do the same thing on one day.
'77 Lund S-16, '94 Johnson 48SPL
'03 Striper 2101 DC, '03 Optimax 150