# Johnson Power boost...20HP to 25HP, maybe more?



## goatchze (Aug 1, 2006)

So a couple of years ago I bought an old beater 20HP Johnson that had been fished off the bottom of Sam Rayburn. It wasn't "functioning" when I bought it, but after a little cleaning/mainetnance I found out the only thing wrong with it was a busted dog clutch.

Motor: $100 on ebay
Dog Clutch: $10 on ebay

So after a $110 investment, I had an working 20HP.

When I bought the motor, I had no idea what kind of shape it was in. Knowing its history, I feared for the worst so I bought the shop manual and a parts manual.

I recently replaced the throttle cam on it and got to looking at the parts manual. I noticed the only difference between the 20HP and 25HP is the carb and intake. Ironically enough, the 20HP has the same pistons/block, etc. as the 30 and 35HP. The only difference besides the carb and intake was the exhaust tube (housings etc. were exactly the same).

So, the way I figure, I can put on the bigger 25HP carb and bore out the intake to match and gain 5HP. I could do the same with a 35HP carb and replace the exchaust tube for a 15HP gain.

Am I insane? Has anyone ever done/heard of this?

It makes me think of the gas tank on my ol' 91 wrangler. It came with two options on fuel capacity, 14 gallons or 20 gallons. In reality, EVERY Jeep came with the 20gallon tank, they just lengthened the vent tube on the 14 gal models to make the pump shut off quicker. (I confirmed this by removing my tube, cutting it 3" and then putting 20 gallons in the tank).

Did OMC do the same thing with the power head on this motor? (early 90s model)


----------



## Mako232 (Sep 16, 2005)

Makes sense if there is no other differences. What about the exhaust?


----------



## REELING 65 (Aug 18, 2009)

I have seen and heard a lot about this. It can be done,but all components must match up for best results.


----------



## goatchze (Aug 1, 2006)

Mako232 said:


> Makes sense if there is no other differences. What about the exhaust?


The exhaust is the same between the 20 and 25HP. It's different for the 30-35HP. In other words, OMC seems to have done the following:

Used 1 power head (block, pistons, etc.) for 20-35HP motors

20HP: base
25HP: add 25HP carb, open up intake to fit
30HP: add 30HP carb, open up intake to fit, add larger exhaust
35HP: add 35HP carb, open up intake to fit, add larger exhaust

From a manufacturing standpoint I guess it makes sense. Still, it makes the bigger motors seem like somewhat of a rip off. There's a pretty big price difference between the 20HP and 35HP with only a handful of parts different.

BTW, I did leave something off. The 30 and 35HP use a different throttle cam, but these are only $7 and it's two screws to replace.

Crazy! Might be worth a try...


----------



## gregr1971 (Oct 28, 2005)

You can try this, but you will find out that you have a motor that's a dog. 
There is much more to it than "just a bigger carb and exhaust".
As with the larger motors with the "same" blocks, there is internal modifications that have been done. The 35 has more aggressive timing, and I'm not talking about ignition timing, I'm talking about the intake and exhaust port timing. 
Now you can go in and attempt to "raise" the ports with a die grinder, but unless you have experience in this on outboards, you will ruin that block. 
In short, it is best left alone.


----------



## goatchze (Aug 1, 2006)

gregr1971 said:


> You can try this, but you will find out that you have a motor that's a dog.
> There is much more to it than "just a bigger carb and exhaust".
> As with the larger motors with the "same" blocks, there is internal modifications that have been done. The 35 has more aggressive timing, and I'm not talking about ignition timing, I'm talking about the intake and exhaust port timing.
> Now you can go in and attempt to "raise" the ports with a die grinder, but unless you have experience in this on outboards, you will ruin that block.
> In short, it is best left alone.


Help me understand better what you're talking about. These are small two stroke motors.

The 20HP and 35HP have identical:
cylinder/crank assembly (OMC #433533)
piston (OMC #396576)
crank (OMC #328374)

How is it that the intake/exhaust port timing can be different? I'm not as experienced with 2 stroke motors as I am with 4 stroke (i have a lot of experience with the internal mechanics there), but I thought on these two strokes the piston and cylinder controled the port timing by their geometry?

Also, I made a mistake in an earlier post. The 25HP and 30HP have identical carb/intakes. The 30HP just has the bigger exhaust.


----------



## Sometimer (Nov 2, 2006)

I think it should work. I've got a buddy who lives in Belton that bought a 24' Blue Wave. He was calling around pricing engines and spoke to a guy somewhere in south TX that told him to save money and get 115 hp Tohatsus (he wanted twin engines on it). The guy modified the carb (maybe something else, too) and turned them into 140 hps for a few hundred bucks. I've ran in his boat, and it runs like a striped ape.


----------



## gregr1971 (Oct 28, 2005)

Port timing acts as the "camshaft" in a 2 cycle engine.
After doing a little research, to make that 25 into a 35, you will need the following:
(based on a '94 vintage motor)
intake and carb for a 35hp
Bypass cover p/n 0436025 
no need to bother with the exhaust.


FYI- the bypass cover on those smaller engines changes the intake port timing.


----------



## gregr1971 (Oct 28, 2005)

NOT going to happen for a "few hundred bucks",especially a pair of them, there is alot more to it than modifying the carbs on the omc crossflow. (i assume we are talking of evinrude/johnson) Unless those motors were put on a dyno and hp increase verified, i will be willing to bet he got a tune-up.



Sometimer said:


> I think it should work. I've got a buddy who lives in Belton that bought a 24' Blue Wave. He was calling around pricing engines and spoke to a guy somewhere in south TX that told him to save money and get 115 hp Tohatsus (he wanted twin engines on it). The guy modified the carb (maybe something else, too) and turned them into 140 hps for a few hundred bucks. I've ran in his boat, and it runs like a striped ape.


----------



## goatchze (Aug 1, 2006)

gregr1971 said:


> Port timing acts as the "camshaft" in a 2 cycle engine.
> After doing a little research, to make that 25 into a 35, you will need the following:
> (based on a '94 vintage motor)
> intake and carb for a 35hp
> ...


Thanks for the advice. I'm in a hotel right now and don't have the parts catalog (the paper catalog makes it really easy to compare the different motors), but after looking at the online one you're right, the inner exhaust tube is the same between the 20 and 35HP. Don't know why I thought it was different.

I'm looking at a 1991 model, the Bypass Cover Assembly is part #433567 for both the 20HP and 35HP, so maybe something changed in 94.

There are a few other parts which are different. I think the 35HP had beefier clamps. But I'm bolted on, so no worries there.

What I'd be curious about, and have no way of knowing without comparing the two, is how much different the intake manifolds are. I've heard it rumored that the only difference is the opening where the carb mounts, the hole being bored out to match the back side opening of the carb. It would make sense to me, as this opening is where the real restriction to flow is on the intake side. For example, I switched out the throttle body on my 2.5L Jeep to that of the 4.0L and opened the intake to match. I definitely picked up several horses (dyno tested and confirmed).

Still, the 35HP carb is about $320 new, so that's a big cost (a new intake is $120). But if I could find a used one it might get done! And in the end, it might be worth the money to almost double the HP!


----------



## gregr1971 (Oct 28, 2005)

what p/n is the carb you want? i just may have one.


----------



## goatchze (Aug 1, 2006)

The 35HP carb assembly is #432701. It shows the same for the 30HP.


----------



## goatchze (Aug 1, 2006)

One difference I did leave off is the prop. If I do decide to go this route, I imagine I'll have to reprop to keep the RPMs down.


----------

