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

[vahunter]

Ive heard the 30/35 intake with more volume doesn't make a noticeable difference. How true that is? Dont know. But if anyone has both intakes laying around plug the carb hole and nipple with duct tape and set it on bench and level the intakes out as close as you can get them. Then fill the intakes with water using a measuring cup keeping track of how much water is in each intake

OR

Level and plug both intakes fill the smaller 20 or 25 intake with water. Carefully lift over 30/35 intake and drain into the 30/35 intake.

THEN

Measure from the mounting surface to the water. This could give you an idea on how much volume difference there is. Maybe a gasket thickness? Maybe fab up a spacer?

Just thinking out loud...

 

[vahunter]

Anyone have accurate sizes on venturi, manifold side, and mouth diameters? I believe 1.47 for manifold side, 1.48 for mouth and roughly 1.25 for the venturi. Im not sure how close the dimensions are. Im going to try boring out my 25hp carb and making a new throttle plate provided I can find a way to remove my idle and main feed tubes without bending them.

 

[Johny25]

Look at the first page closely and the pic of the 25 intake next to the 30/35 intake. Roughly a 1/4" larger opening all the way around. You can see the gasket line showing the size difference. Not to mention it is a deeper intake. It is a considerable volume increase considering the small size of the intakes to begin with

 

[foxhunter]

i just did this conversion on my 1985 20 hp, the difference in the hole shot was like night and day. propped the moter to 5500 rpm and top speed went from 18-20 mph with stock 20 hp to mid to upper 20"s. the acceleration is awsome.
i have a converted 20 hp inatke to a 1.5" hole if someone needs one.

 

[vahunter]

FYI:
I did a test with my bigger carb with stock 25 intake and the bigger 30/35 intake. I saw a very very minor change. So IMO im not sure the bigger volume intake matters as long as the hole size matches the carb.

 

[Sportsman570]

I have found this tread to be very informative and thank Johny25 for posting in such detail. My situation is slightly different from most as I'm looking to mod a 1988 Mil. surplus, Johnson 25Hp(AEM25B). It has the same 31.8 Cubic inch power head as ones already discussed. The intake manifold is PN 396788 with the Carb assembly being PN 432168.

As several of you have already done, I'd like to squeeze more out of the outboard and believe that I have understanding of what's required. My intention is to replace existing carburetor with a new or rebuilt Carb. assembly PN 0432021, and to replace the existing intake manifold with PN-0396790. Does this sound correct?

Also, I'm a little confused when it comes to the intake manifolds and their part numbers. I'm having a little trouble locating a used 0396790 intake but have found a used 0391054 for sale. From what I understand the 790 replaced the 054 model. Other than the part numbers are these intakes identical in every way?

Should I pass and keep looking for used 0396790 or quit being so damn cheap and buy new? I wouldn't think a new manifold would perform and better than a used one that was in good condition but I could be wrong.

 

[rusty503]

You need to check eBay. There is one up there now. Drop the leading 0 and search for 396790.

 

[Sportsman570]

rusty503 » 03 Jul 2014, 21:52[/url]"]You need to check eBay. There is one up there now. Drop the leading 0 and search for 396790.

Johny25 are you referring to:

https://www.ebay.com/itm/Johnson-Evinrude-OMC-25hp-Intake-Manifold-P-N-396788-396790-/261353743027?pt=Boat_Parts_Accessories_Gear&hash=item3cd9e5a2b3&vxp=mtr

They listed both 788 and 790 part numbers. But zooming in it is stamped 331454. I'm confused...

 

[Johny25]

I can tell just by looking at the picture on ebay that....that is a 20-25hp intake, not the 30-35 you are looking for. And yes the 0391054 is the same thing as the 0396790. I am sure that the 0391054 is what I had on mine (sold boat and all last week). The 0396790 just replaced the 054 number. If you look at the pic of the back side of the intake compared to the pics on the first page of this thread that I posted you can see it is the smaller intake

 

[rusty503]

Doing another search on eBay I found the 391054 one you want. Do a search for 391054 leaving off the leading 0. One picture even shows it stamped with the correct number.

 

[Sportsman570]

Thanks; I just ordered it. Now I guess I need to start looking for a carb. Are there other part numbers other than 432021 that I should be looking for?

 

[rusty503]

Quick search shows that part number has been replaced with part number 0439389.

 

[rusty503]

Guess what? There is one of them on eBay too.

 

[Sportsman570]

Before I pull the trigger and purchase, I have a few questions. The carb. looks like it has a bracket for an electric choke. My current engine does not have a choke, would this pose any issues (especially starting)? Also, in the photo there does not appear to an adjustment knob for the slow speed needle valve. Do you just use a screwdriver or could this be an issue? One final question, the carb on ebay has a Throttle control cam beside it. Will the carb swap also require a new Cam?

 

[rusty503]

Go to https://www.potomacmarine.com/pages/Oemparts?airbrand=EVI#%2fEvinrude%2fE30TELCCE_1988%2fCARBURETOR%2f27426%2f46344 and check it out there. That is where I got the part number.

 

[Sportsman570]

Johny25, If I'm unable to find a donor carb (as you did), without a choke, do you see an issue with using a 35hp carb such as part number 0439389 and disabling the choke by "locking" it's flapper open or somehow removing it altogether. Remember, my current setup uses a primer vice choke.

 

[spcamno]

Hi Johny25,

Thanks for making this very informative post!

I have some questions for you may be you can help me out.

I have a 1990 Evinrude 20hp (E20CRESB) and when I compare the parts diagram it does show a different carb to a 35hp with same model year and what exactly do I need to get in order to increase the output?

Should I get a carb and manifold from a donor 35hp or open up a restrictor plate mentioned in another thread?

Also, is the 40hp share the same head, block etc and what is the maximum possible I can get this thing to?

Thank you in advance!

 

[Johny25]

Do not think they made a 35hp in 1990.....and if they did it was likely a restricted 40-50hp. No the 40 and 50 had a larger and different block entirely...44 or 45ci I believe

 

[spcamno]

Johny25 » 22 Jul 2014, 12:01[/url]"]Do not think they made a 35hp in 1990.....and if they did it was likely a restricted 40-50hp. No the 40 and 50 had a larger and different block entirely...44 or 45ci I believe

Thanks Johny25 for your reply.

After some research on the internet my motor is a 1989 not 90 and found info on iboats someone did a mod on a same model 89 by just open up the restriction plate.

Can you tell me if it this the only thing difference between the 20 and 25?

I remember read something about the cam on the throttle or flywheel to limit the rpm and the carb and manifold is bigger, is that true or only apply to the 30 or 35?

Thanks again for your help.

 

[Johny25]

the only difference on between the 20 and 25 in that year is the restricted intake hole from everything I have seen on the motors that I have had