Converting a 20 HP Johnson to 35 HP

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

If ya wanta get with it, just file out the intake mfld. on a Belgian OMC 6, boost it to 7.5 hp. Vroooooooooooooooooooommmmmmmmmm! Why that? Because in Europe you can't use over 5 hp (measured at propshaft) without passing a written driver test (bureaucratic legal language, ship signals, flags, worse ...) that requires more prep time than a Ph. D. exam. My wife and I just passed it, and I wouldn't want to go through it again. So my Belgian "6" will soon be for sale, we've rebuilt a 1981 15 and are rebuilding a 1978 25 (will be a 35, of course) and 1980 35 as well.

This is a very interesting post. Kind of reminds me of hopping up an old Mopar car engine, match porting the intake to the heads, adding headers, bigger carb, etc...

Great replies!

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

Such modifications are strictly forbidden to Canadian fishermen!

Hi there. So if I am reading your post correctly...I should be able to take my 1981 johnson 20, get a 35 hp carb, use the hole enlargement method with your wood jig etc, and it was recommended to swap to a 15 inch prop to keep the rpms reasonable.

That sounds very easy, and cheap, for a whole lot of extra power.

Fix it, can you verify your wot speed, and also your trolling speed?

Does doing this make trolling at reasonable speeds out of the question?

Best Regards, and this is a totally awsome thread. I'm going to do this to my 20. ryan

 

[ryanmitton]

Re: Converting a 20 HP Johnson to 35 HP

@ Sandhammaren:
Wow. You are a wealth of knowledge that's for sure. I see from your signature that you 'rebuild omcs for relaxation'. Very cool.

While Fix_It (original poster) converted a johnson '87/'88 20 hp ---> 35 hp carb mod, myself I am starting with a johnson '81 20 HP (and actually I have a second 1982 that will also get the "upgrade" if it works). I beleive that Larlarsa also has the same '81 that I do.

Here is one of your quotes which make me think this would be *not* a good idea for us with our 1981' 20's.

"No, the 1985 and later 20 & 30 gearcases are identical with the 1977 and later 35 gearcase. Likewise for the 1985 and later 25 gearcase. However, the
1976-1984 25 gearcase is much smaller (bullet is bolted together from 2 halves) and us geared higher, making it faster (less underwater drag from both gearcase and prop). The problem is the only props that fit the 7/8" dia. 13 spline propshaft are fishing props. For the 'sprint' gearcase (the standard 20,30,35 gearcase) you can adapt 10x14-10x17 Yamaha cleavers (but without rubber hub)."[/I] (so this tells me that our 1981 (and the 2nd '82) 20's have the smaller gearcase that is geared higher. Which would mean that doing the 35 hp carb modification would likely compromise the gearcase as it will spin even faster. Furthermore, the restriction of prop sizes to "fishing sizes" mean that we cannot "slow down" the rpm of the prop by simply putting a larger and higher pitch prop on.)


So Sandhammeren is my logic correct? At the end of the day, is it safe for us to do the 35hp carb mod to our 81 (and 82!) 20 hp johnsons?


Thank You for providing the link to the parts manual. Interestingly, I don't even SEE a place for my '81 20 hp, so I can only assume that the parts must be identical to the '81 25hp and they want us to use the 25 hp diagrams and part numbers to order new parts.

-------------------------

Ryan.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

I wouldn't troll with a 20-35 hp motor, only adds carbon to the rings. Get an old Lightwin (40 lb or less) or 5 hp and clamp it on beside the bigger motor. I've got a 1977 4 hp Lightwin hanging on my garage wall as decoration.

10x15 prop would be for a boat that runs around 40 mph or better, too much pitch for carrying a load. There's a video of a short Lowe al. boat that really trucks with a ca. 1946 Mercury 7.5,

http://www.youtube.com/watch?v=uexj2pwpK-o&feature=related

would like to try one with my 1981 OMC 15. wouldn't mind having that old 7.5 either, I used to repair the more modern KG-7s as teenager in the late fifties.

Hi there. So if I am reading your post correctly...I should be able to take my 1981 johnson 20, get a 35 hp carb, use the hole enlargement method with your wood jig etc, and it was recommended to swap to a 15 inch prop to keep the rpms reasonable.

That sounds very easy, and cheap, for a whole lot of extra power.

Fix it, can you verify your wot speed, and also your trolling speed?

Does doing this make trolling at reasonable speeds out of the question?

Best Regards, and this is a totally awsome thread. I'm going to do this to my 20. ryan

 

[Whoopbass]

Re: Converting a 20 HP Johnson to 35 HP

Just to confirm, this is the carb you bought for yours right?

http://cgi.ebay.com/ebaymotors/Johnson-Evinrude-35-HP-Carburator-and-Cover-Carb_W0QQcmdZViewItemQQ_trkparmsZ72Q3a1205Q7c66Q3a2Q7c65Q3a12Q7c39Q3a1Q7c240Q3a1318Q7c301Q3a1Q7c293Q3a1Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem4a97da4e50QQitemZ320375246416QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

I was watching this carb on ebay before you posted it here. Now its been bid up to $116.00. Before this carb was listed they were selling for $30-$60 recently on Ebay so whoever is bidding this amount is crazy.
I will wait.

 

[Sandhammaren05]

Re: Converting a 20 HP Johnson to 35 HP

I've watched prices since last Aug., when I became aware that the 20,25,30, and 35 powerheads are all used in GT-Pro class racing, the 20, 25 and 30 with only carb change. In warm weather the carbs went on Ebay for as high as $200. I've paid $30, $50, not more during the winter. To get one cheap you need to wait til next Jan. when the supply's up and the demand's down. Otherwise, expect to shell out $200 (cheap for 10-15 hp increase). A new carburetor probably costs $300, you can get a rebuilt one from Stone & Son in Houston (huge parts warehouse for older motors) for $175.

PS Mine are not for sale, I'm rebuilding 25s and 30's to 35s.

I was watching this carb on ebay before you posted it here. Now its been bid up to $116.00. Before this carb was listed they were selling for $30-$60 recently on Ebay so whoever is bidding this amount is crazy.
I will wait.

 

[Ober51]

Re: Converting a 20 HP Johnson to 35 HP

I havent read through everything yet - but would it be possible to use a 1988 35 HP carb on my 25 HP 1980, both Johnsons?

 

[fastfrankie]

Re: Converting a 20 HP Johnson to 35 HP

can i do this with a1991 20 hp ? thanks frank

 

[Whoopbass]

Re: Converting a 20 HP Johnson to 35 HP

See post #35

 

[fix_it]

Re: Converting a 20 HP Johnson to 35 HP

Sorry guys. Haven't been on here in a while. Kinda been busy. Haven't had much time to play with the boat. Anyway, I did test the speed with a GPS, and found my speedometer to be inacurate. No surprise here. But I have forgotten the speed that I measured. I believe it was in the mid 30s, maybe.

As for the trolling speeds, I can idle slowly and troll very well. But I don't do it often because of the build up of carbon running so slow.

When modifying the intake, I removed it from the engine. Do NOT enlarge the hole with the manifold on the engine. You WILL get shavings in the motor. You do not want that. It is not that hard to take it off. I even reused the gasket.

I am happy with the mod, and I would do it again if I had the need. I will try to check the GPS again next time I go out.

 

[samadhis]

Re: Converting a 20 HP Johnson to 35 HP

I try this modificationand and it's work great.

I bought a use intake manifold on ebay (so no need to enlarge the hole) and I found a 35 hp carb.

The result:

1 person and fishing stuff: before 20 mph, after 25mph

But the most interresting

2 persons and fishing stuff: before 11 mph, and after modification 22 mph.

(all speed is with the same propeller 9 1/2 x 11)

 

[outkast7222]

Re: Converting a 20 HP Johnson to 35 HP

This thread caught my eye because I had a 20 on my old boat and the new one has a 35, but what gets me is everyone is talking about enlarging the hole on your intake. The hole on my intake is smaller than the opening on my carb, so if I enlarge it to match the carb I would have something larger than a 35?

Also does anyone have the actual measurement of the hole afterward? I want to say mine is just a hair under an inch.

 

[wbeaton]

Re: Converting a 20 HP Johnson to 35 HP

before 20 mph, after 25mph...(all speed is with the same propeller 9 1/2 x 11)

With a 1987 20 hp my boat went 20 mph. With a 1988 30 hp it goes 26 mph. Same prop as yours.

No doubt there is an increase achieved, but I still don't believe you are getting 35 hp. Sounds more like 25-30 hp, which was my original guess-timation. However, its a successful mod and you should be proud. Congratulations.

 

[martijn]

Re: Converting a 20 HP Johnson to 35 HP

hey i'm new here .

today i took two johnson engines apart .
a 25 johnson and a 35 johnson, both 1979 models .
the 25 has a cavitationplate exhaust and the 35 has a through hub exhaust .
to clear some things out :

the exhaust pipes are the same (both same size )
the through exhaust lower part can be interchanged with a cavitation exh one
the carb from the 35 is non restricted. (no nylon ring )
the 35 has a bigger prop (with bigger spline shaft )
the drive axle from the 35 fits the 25 crankshaft .
the engine block is exactly the same on 25 and 35 (ports etc)
the ignitionplate on the 25 goes further than on the 35 hp (stopper bolt )
the clamp system from the 35 is bigger than the 25
the clamp system can be interchanged between 35 and 25 hp
the intake plate holes are the same on 25 and 35 hp .

i hope some questions are answerd .
greetz from the netherlands .
martijn .

 

[Chinewalker]

Re: Converting a 20 HP Johnson to 35 HP

Also, keep in mind that not all 20s, 25s, 30s, an 35s are created equal. The big block 20 came about after the horsepower ratings were moved from powerhead to propshaft. It is essentially the pre 1985 25hp, with through-hub exhaust. The 35 became the 30, and so forth. Same motors, differently rated... Soooo, the bottom line is that you cannot convert a 20 to a 35. Period. Best you can do is a 30, if you're comparing apples to apples.

 

[AZcamperguy]

Re: Converting a 20 HP Johnson to 35 HP

subscribing to this very interesting thread. I purchased a used Johnson 20hp and was told it was "bored out to a 30hp".

 

[Tolly family]

Re: Converting a 20 HP Johnson to 35 HP

I have a 35 carb cleaned up and ready to install on my 28SPL but have a question about the choke/primer. My 28 had the electric primer and the new 35 carb has the electric/manual choke assembly. Which one shoul I use? The original primer should be easy to keep but the choke may be easier to live with as it has the manual pull knob included and could be used during warm up easier than pumping the key to activate the primer. Let me know what you have done.

Thanks,

Dan

 

[Chinewalker]

Re: Converting a 20 HP Johnson to 35 HP

Dan,
Your 28 is already equivalent to the old 35. The 28s came out after the horsepower re-rating. The 28 is actually the 30hp powerhead (same as pre-1985 35) mated to the old style 25hp lower unit. They called it a 28 SPL simply to differentiate it from the more fully featured 30. Adding an earlier 35 carb will gain you nothing...
- Scott

 

[Tolly family]

Re: Converting a 20 HP Johnson to 35 HP

Oh bummer after I got the carb and rebuilt it! My 28SPL is a 1989 I converted to a short shaft and put on a 11' Whaler, it runs 30 mph at 5700 RPM with a 11P spline drive prop. This thing runs sweet but is a huge fuel hog.

Dan

 

[jimmbo]

Re: Converting a 20 HP Johnson to 35 HP

hey i'm new here .

today i took two johnson engines apart .
a 25 johnson and a 35 johnson, both 1979 models .
the 25 has a cavitationplate exhaust and the 35 has a through hub exhaust .
to clear some things out :

the exhaust pipes are the same (both same size )
the through exhaust lower part can be interchanged with a cavitation exh one
the carb from the 35 is non restricted. (no nylon ring )
the 35 has a bigger prop (with bigger spline shaft )
the drive axle from the 35 fits the 25 crankshaft .
the engine block is exactly the same on 25 and 35 (ports etc)
the ignitionplate on the 25 goes further than on the 35 hp (stopper bolt )
the clamp system from the 35 is bigger than the 25
the clamp system can be interchanged between 35 and 25 hp
the intake plate holes are the same on 25 and 35 hp .

i hope some questions are answerd .
greetz from the netherlands .
martijn .

The lower units will not swap on a the 76 thru mid 80s models. http://rides.webshots.com/photo/2609670230101354590ozLxGB The bolt patterns are different as are the midsections. Also the Cylinder heads are different.