Converting a 20 HP Johnson to 35 HP

[warpedrotor]

Re: Converting a 20 HP Johnson to 35 HP

I just purchased a 1978 Evinrude 20 model 208cr02. It has the smaller foot that separates into two halves.This model is not listed anywhere except I have found on some sites that the CR stands for 'Canada Derated'. If I look down the bore of the carburetor with the throttle valve open , I can't see any restrictions caused by a smaller intake opening. All sites selling parts list only a 25 hp for this year for Johnson and Evinrude. Can anyone tell me the difference between the carburetors from 20 to 25 hp like a bore measurement etc? I'm thinking I already have a 25 hp, it had just been badged differently for sale in Canada and I want to confirm.

 

[imalunatwo]

Re: Converting a 20 HP Johnson to 35 HP

What would i need to convert my 82 25hp model 25telcnb to a 35 hp? Is it just a carb change or is there more to it? Thanks

 

[warpedrotor]

Re: Converting a 20 HP Johnson to 35 HP

I was finally able to try my '78 Evinrude 20; with me at 180lbs, my friend at 275lbs on a 225 lbs fiberglass boat (Caderette) we were able to hit 35 km hr, the motor was revving high. Alone I was only slightly faster, but planed and got up to top speed sooner. So with the smaller fisherman prop it's probably not any use changing the carburetor after all!

 

[montff77]

Re: Converting a 20 HP Johnson to 35 HP

Hi. Brand spanking new here!!!! Ok I know the post title, BUT I can't find the info I need to convert my 1964 18hp to a 20hp (earliest 20hp was 1966). I know it isn't a big gain but its 11%!!! From what I gather on schematic its just reed stops that differ? Anyone confirm or deny this? Once I perform this mod i will test for max speed and rpm increase. Then ill install the head I've had shaved .060 and recheck. That would have to bring me to a 25??? I have to replace the head anyway bc mine has a threaded insert that is leaking and partially stripped out so the extra effort isn't that much. I've been told to square the ports and blend the transfer area as well any thoughts are SO GREATLY appreciated. Especially if its what I want to here! Lol, thanks yall. This has been good reading and I have my eyeball out for a 70's to early 80's 20hp now

 

[1946Zephyr]

Re: Converting a 20 HP Johnson to 35 HP

It's in the intake manifold and carb. I love it....an old thread that has been revived several times over the last three years. LOL

Don't bother doing anything to the head. You'll only screw up your motor. The 25 has a different crank and even a bigger carb yet, than the 20 plus there are differences in the pistons and exhaust. If you want a 25, then I would suggest going out and buying a 1969 -1972 model. It has the old style ignition, which is inexpensive to maintain and the less weight for the horsepower.

 

[montff77]

Re: Converting a 20 HP Johnson to 35 HP

Thanks for the reply. My point was easily missed bc I've kinda hijacked this post bc of the knowledgeable posters in it. "What do I have to do to change my 18hp into a 20hp" am I correct in that it is only the reed stops? The parts diagram show the same carb and intake..... So that's 1 no for shaving the head :-/

 

[LakeRacer99]

Re: Converting a 20 HP Johnson to 35 HP

Exactly as I said.

What might lead to a problem is to do what I'm about to do, namely, put a 35 carb on a rebuilt 25 powerhead that came with the old style small gearcase (13 spline prop shaft, gearcase looks otherwise like the 1972 shearpin model). That gearcase is smaller and higher geared, is therefore faster, but will wear out (or crap out) quicker with 35 hp than the more modern ('sprint'-type) gearcase on the 20.

Joe:...can the splined propshaft be swapped in place of the shearpin propshaft on the smaller 2 piece lowers? I can't tell is that is what you are saying here.
thanks,

 

[sprozen79]

Re: Converting a 20 HP Johnson to 35 HP

I have found all this very helpfull, I have just started clubmans boat racing. I have a 1974 25hp motor. I no it doesnt make 25hp as the newer ones do.... so... I need more power to compete with the newer motors. Am I able to simply do the carb replacment to bring it up to a 25hp or higher?

 

[wbeaton]

Re: Converting a 20 HP Johnson to 35 HP

Thanks for the reply. My point was easily missed bc I've kinda hijacked this post bc of the knowledgeable posters in it. "What do I have to do to change my 18hp into a 20hp" am I correct in that it is only the reed stops? The parts diagram show the same carb and intake..... So that's 1 no for shaving the head :-/

There is no difference between the 18 and 20 hp. Chalk it up to marketing. You wouldn't notice the additional 2 hp anyway. Also, the 25 hp of this series was underpowered. Most complain they can't tell much difference between the 20 and 25 hp either. The later 20-35 hp motors that this post was based on are much bigger displacement than the early 18-20-25 hp.

 

[montff77]

Re: Converting a 20 HP Johnson to 35 HP

I see. It's just hard to believe 2 HP wouldn't be felt but I have no experience to rely on, just imagination and help from this site. I know that the 3 you mentioned are on a 22ci block and the later on a (roughly) 31ci block. Also older motors measured HP at the head and not the shaft like later ones. I guess I'm just Hung on converting because only the reeds differ. I still say that If I have to tear it down ill go back with the 20hp stops. I'll soon have a tach and would be able to note any difference. Thankyou for replying.

 

[srimes]

Re: Converting a 20 HP Johnson to 35 HP

Completely possible. Nothing different in the exaust. I did it on a 1984 20hp to a 30hp. On my 13 pitch prop I went from 5300 rpm to over 6300. Had to move to a 15 pitch to get rpm to 6k.

Boat topped out at 37 MPH by GPS Max theoretical speed was 41
The boat would scoot. I ran off and left many heavily load runabouts on lake Degrey. But very careful and always wear your kill switch.

My 14x36 ran basically on the prop.
Sold it to a young man from Rodgers. Be very careful.




Kyle

That's great. More specs on the boat please? Was it just a bare hull tiller steer, or decked out at all? I have a 14' grumman w/ decks, tm, side console, etc. and a 25hp johnson and I'm wondering what kind of speed I can get out of it. My motor is a 97, can it be upgraded?

 

[headdamage]

Re: Converting a 20 HP Johnson to 35 HP

This is a great thread! I'm looking for a OMC 35hp for my Mk2 zodiac but haven't found any for under $2000 but I have found a good looking 1988 20hp Evinrude for $950. I'm thinking of picking it up and doing the 35hp carb conversion on it later.

 

[1989sho]

Re: Converting a 20 HP Johnson to 35 HP

I just purchased a 1978 Evinrude 20 model 208cr02. It has the smaller foot that separates into two halves.This model is not listed anywhere except I have found on some sites that the CR stands for 'Canada Derated'. If I look down the bore of the carburetor with the throttle valve open , I can't see any restrictions caused by a smaller intake opening. All sites selling parts list only a 25 hp for this year for Johnson and Evinrude. Can anyone tell me the difference between the carburetors from 20 to 25 hp like a bore measurement etc? I'm thinking I already have a 25 hp, it had just been badged differently for sale in Canada and I want to confirm.

I have the same motor and can't allways find it listed in parts websites. Still not sure if it is the same as a US 25. Anyway I get 30 mph on a 10' F/G boat,26 mph with my9.9 with a 15 hp carb. So not sure yet if any mods are possible.