80's johnson/evinrude 25hp to 30-35hp conversion with pics

[Cab1040]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

ok thanx glad i waited till this weekend to change it.. well if you know where are anyone that has a 35 intake and carb forsale im interested

 

[Johny25]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

Sent you a PM Cab1040

 

[Chevdog]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

I am looking to do this 30 Hp mod to my 93 25hp johnson. I have found a carb for $100 dollars but don't know what intake manifold I need. There are 3 listed that fit my motor but none labelled for a 30 Hp carb.
The carb number is # 435376, but like I said I can't find the correct manifold for it. If anyone has the part number or has one laying around I would be more than happy to get it. Also, the same carb is used for the 30-35 Hp motor, what sets the two apart other than the carb?
Thanks, Josh.

 

[Johny25]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

I am guessing the 30 and 35hp are separated only by Intakes. There are 3 intakes like you said for that year. I know the 0396788 is NOT the one you want. I believe the 0434148 is going to be for the 20hp and possibly 25hp version, just a different carb separates the 20 from the 25. The 30hp probably has the 0396788 intake with the same carb as the 35hp. And the 35hp most likely has the 0396790 intake which separates it from the 30.

Does this make sense? I believe you want the 0396790 intake. What is the intake number you have on your 25hp, this may help me confirm the differences in the 4 versions.

 

[Chevdog]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

I will have to look for the number tomorrow since the motor is at my work. I tried to find it yesterday but couldn't find a number on the intake. Do you know where the number is stamped into the intake? I don't really want to take it off since im more than likely going to use the boat before I can gather all the parts for this.

 

[Erik_Scarab]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

Hi Johny25 and others interested in this topic.

Johny25, I am really impressed by the work you have done on your engine!

Back to the 70's and my secondary boat, my father bought as new an 1978 20 hp Johnson that has worked as a swiss watch the last 35 years, the glassfiber boat it was on is now scrapped! The replacement boat (inflated 5 meter / 15 feet long) is a little bit longer and I bought a used 35 hp carb from a 1981 Johnson to give the boat a better push. I have not installed it yet, and have not yet checked compatibility.

Do you have any knowledge if the Johnsons from 1978 need a new or modified intake, or other work than described in this thread? My local mechanic said that it will never work, different exhausts pipes etc...(But he wants to sell me a new engine from his store..)

You have obviously made it with the engines from the 80s without modifying the exhaust pipes.
Adding an image of the engine in order for you to see which generation I am taking about.

 

[Johny25]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

Thanks Erik : ) Ok first thing we need to do is verify the year and model of your motor. From my info it says that the 20hp was discontinued from 1974-1984. So 1973 being the last year in the 70's and 1985 being the first year back in the 80's. The confusing thing is that is definitely 1978 decals and cowling yet Johnson supposedly did not make a 20hp in 78'. There brochures say they only had a 25 and 35 in 1978. So the question is...... what is it? Did someone put aftermarket 20hp stickers on it?
Post the model and serial numbers from the transom tag or welch plug on the block and we can go from there.

But the difference betweent the 25 and 35 in 78' was carburetor, intake, gearcase (25 had smaller split case and the 35 had the larger solid case) and the exhaust housing assembly had different parts on it. The exhaust tube was identical though. The 35hp in 78' had a larger more beefy transom mount with different little parts on it but none of it was for performance increase, just a heavier setup. I have seen people still do this mod with the smaller gearcase and have had no issues. Also I run the smaller transom bracket assembly on my 88' and have no issues either.

I can't see clear enough in your pic, but do you have the split gearcase?

 

[Erik_Scarab]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

I am located in Sweden (Europe) and maybe the product range was different here than in the US. Or maybe the change from hp-rating on propeller vs. powerhead was done at another year in Europe, and my motor being a 25 hp powerhead, measuring up at 20 hp on the propeller - I do not know.

The details on the engine are as follows:
My model no. is 20BF78C
Serial no. is B 2481

I do not remember if it is the split gear case or not and the motor is in another part of the country, cannot check it now.

 

[Johny25]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

Yep that is surely what it looks like. The BF stands for Belgium Derated model. Which tells me that it is a restricted 25hp. So it does look like a European 1978 20hp. The only thing I believe that you have to do to turn it into a 25hp is to bore out the intake manifold hole to match the hole from the backside of the carb where it mounts to the intake. My guess is if you look down the carburetor with the choke open you will see an intake hole about the size of a quarter, very small. This is how they restricted them. And it most likely has the split lower gearcase, as all the 25's of that year did.

Does your motor look like this by chance?

 

[Erik_Scarab]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

Yep that is surely what it looks like. The BF stands for Belgium Derated model. Which tells me that it is a restricted 25hp. So it does look like a European 1978 20hp. The only thing I believe that you have to do to turn it into a 25hp is to bore out the intake manifold hole to match the hole from the backside of the carb where it mounts to the intake. My guess is if you look down the carburetor with the choke open you will see an intake hole about the size of a quarter, very small. This is how they restricted them. And it most likely has the split lower gearcase, as all the 25's of that year did.

Does your motor look like this by chance?
View attachment 166148

No, that is another graphical layout.

Here is a picture of how mine looks like (it is not the actual engine, I have borrowed it from an ad of a 25hp 1979, but it looks identical to mine.)


To sum up:
Option 1 - I can bore out the intake, and I will have 25hp instead of 20hp? Easy, and I will probably try this first.
Option 2 - I can clamp on the carb and intake from a 35hp that I bought of Ebay from the same year (a US-made 35 carb on the Belgian-made 20-25)?

A side question - what does usually/finally stop these engines? Mine is 34 years old and still works as it did when I was a child. Do you have any feeling of the 35hp-versions having a shorter life than the obviously "under-powered" 20hp-versions?

 

[Johny25]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

Option 1: Yes, if you have the small restricted intake then this is all you have to do to get the 25hp out of it.
Option 2: Yes, there should be no difference in the American or Belgium models (or not enough to make a difference). I had a Canada derated model at one time and it was no different than the American version.

The side question: This is a loaded question because it all depends on how the motor is treated and maintained. Run 50:1 mix and NOT the 100:1 that the factory sticker may say. This will increase longevity along with basic general maintenance and care of the engine. But if you wonder what finally happens to engines that are very well taken care of the answer is low compression in the cylinders due to worn rings and etc. Basically the motor wears out internally at some point no matter what you do. But there are many many of these motors like yours still running strong today and will continue too for many years to come. Just maintain your engine properly and it could outlast you (depending on your age lol). This OMC 31.9ci size motor is well known for being a strong long lasting workhorse, some say bullet proof. This was the golden age of OMC and was the best in the business in this era.
Yes the 35hp will or should not last as long as the 20hp. The engine is going to turn 500-800rpm higher with the larger carb and intake so physics says it will wear out faster just do to that alone.

 

[JussiT]

Re: 80's johnson/evinrude 25hp to 30-35hp conversion with pics

Hi
I have 1990 Johnson 30 BJ30ELESS. My boat weights total with me around 850lbs. Prop is 10x15 and my top speed is 32mph. I think speed is quite good. With total weight of 1200lbs speed drops to 27mph.
Winter is coming and i have been thinking about maintenance of the motor. For this reason i was reading parts numbers and generally looking around my outboard. I found that my intake manifold diameter is 1.248 inches. Carburetor diameter fits with intake manifold. Venturi size is 0.897. Upper orifice plug has mark that says it is 0.30. I compared measurements with your projects and now i am wondering if my carburetor is too small for johnson 30.

My measurements in SI "Not so accurate"

Topspeed 52 km/h "GPS"
Weight estimated 385 kg
Intake manifold 32mm
Venturi size 23mm

PS. Sorry for my grammars. It has been about 15 years since my last english class.