Johny25
Well-known member
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?)
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.
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.
Here is a pic of the 35 in front and 25 behind it to show the size difference again.
And a couple pics showing the difference in width
The 35 at 5 and 5/16
And the 25 at 5 and 1/16
Here is the intake hole differences.
The 35 at 1.5 inches
And the 25 at just over 1 and 3/8 inches
And here is the 25 carb mounted on the 25 intake
And the 35 carb on the 35 intake is perfectly flush with no flow restriction
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
And the 25 nipple which you can barely see in the pic
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)
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?)
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.
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.
Here is a pic of the 35 in front and 25 behind it to show the size difference again.
And a couple pics showing the difference in width
The 35 at 5 and 5/16
And the 25 at 5 and 1/16
Here is the intake hole differences.
The 35 at 1.5 inches
And the 25 at just over 1 and 3/8 inches
And here is the 25 carb mounted on the 25 intake
And the 35 carb on the 35 intake is perfectly flush with no flow restriction
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
And the 25 nipple which you can barely see in the pic
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)