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

[erikhill]

Hey Johny25, What is the part number for the 35hp carb you used? I just installed the larger intake manifold on my 20hp with the stock carb and noticed a 3mph difference on the water. I still want to put on the larger carb, but I'm having a hard time locating one.

Thanks,
-Erik

 

[spcamno]

Johny25 » 24 Jul 2014, 23:20[/url]"]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

Thanks for your prompt post!

I totally missed this reply until erikhill's post.

It must have gone to my junk mail folder.


Thanks again will report back after I open up mine hopefully next week or so.

Cheers

 

[alfaguy]

I am upgrading my 1988 25hp to 30 and bought an early carb with a choke plate. I am wondering if I can make a hybrid carb with the bottom from the 25 and top from the 30? I would have to insert a primer fitting in the old carb behind the throttle plate.

Probably safer to use the choke system.

Dave

 

[vahunter]

I think the primer fitting on manifold side is pressed. I don't believe its supposed to be removable could be wrong. By "bottom" do you mean the bowl of the carb? I changed the bowl on my carb and honestly I think youd be better off swapping the plug for the hose nipple and of coarse the high-speed jet/orifice. You will need a special screwdriver to change the hi speed jet though. I made one by grinding the diameter down on a flathead screwdriver so it would fit the screwdriver slot well without damaging the threads in the bowl.

 

[alfaguy]

I received a suggestion to drill my early carb on another post and press in the primer nozzle tube. This sounds like a good idea if I can do it successfully. Has anyone else tried this??

 

[alfaguy]

I installed my 30/35 hp choke type carb on my 25 and she cranked right up. I was surprised to find my intake manifold has the larger hole to match the bigger carb, so the manifold I bought is not needed.

I am headed to the river tomorrow for a long weekend of hunting and fishing. A great test and fine tuning opportunity,

Thanks to the contributors of this post who offered encouragement - You can dooo it.

 

[jasper60103]

I installed my 30/35 hp choke type carb on my 25 and she cranked right up. I was surprised to find my intake manifold has the larger hole to match the bigger carb, so the manifold I bought is not needed.

I am headed to the river tomorrow for a long weekend of hunting and fishing. A great test and fine tuning opportunity,

Thanks to the contributors of this post who offered encouragement - You can dooo it.

Great. Keep us posted.

 

[shawnfish]

Nice video of the old 20 screaming around......although my head is now kinked lol. The 72' you had was the small 22ci block which was used on the 20 and 25hp's up until 77' I believe when the bigger 31.8ci block began being used. Those old 22ci motors were very light and had a great power to weight ratio......think that motor you had was likely less than 85lbs as opposed to the 80' you have that weighs 30+ pounds more I would guess

j25rcs 104lbs
j25rlcs 107lbs
j25tecs 113lbs
j25telcs 116lbs

 

[shawnfish]

i had a older 72 johnson 20 hp. i stuck a 25 hp carb on it and it flew. it was on a 14 foot duroboat and at wot it was scary fast. sold it to a guy who is still running the motor. i didnt have a gps, but, it planed in seconds but would chine walk at wot.

https://www.youtube.com/watch?v=E5_RQaHvx4Q

i now have a 80 johnson 25, that needed a lot of work. i got it back together and am going to swap out the intake and carb (if i can find a set cheap) . motor seems to have way more power than the older 72 but still has the little split case lower unit. im thinking the lower should hold up as the 50's motors (30hp) had the same style lowers.

here she is running. i have since installed a lanyard style kill switch (missing in the vid, thats the loose wire)

https://www.youtube.com/watch?v=sySIsymJeSQ

thanks johny for inspiring us with your great thread,
dino

1980 25's and 35's have same intakes but different carbs so only carb needs replaced.. I have a 1980 25 with 35 carb on it...

 

[shawnfish]

i had a older 72 johnson 20 hp. i stuck a 25 hp carb on it and it flew. it was on a 14 foot duroboat and at wot it was scary fast. sold it to a guy who is still running the motor. i didnt have a gps, but, it planed in seconds but would chine walk at wot.

https://www.youtube.com/watch?v=E5_RQaHvx4Q

i now have a 80 johnson 25, that needed a lot of work. i got it back together and am going to swap out the intake and carb (if i can find a set cheap) . motor seems to have way more power than the older 72 but still has the little split case lower unit. im thinking the lower should hold up as the 50's motors (30hp) had the same style lowers.

here she is running. i have since installed a lanyard style kill switch (missing in the vid, thats the loose wire)

https://www.youtube.com/watch?v=sySIsymJeSQ

thanks johny for inspiring us with your great thread,
dino

1980 25's and 35's have same intakes but different carbs so only carb needs replaced.. I have a 1980 25 with 35 carb on it...

and your question about the lower holding up... yes, yes it will. the 50's thru 80's johnsons were built very very tough and the 25 lowers will handle the xtra hp, mine has with no problems whatsoever...no metal flakes in the lower oil when I have changed/checked mine....

 

[shawnfish]

johnny25 gave me the idea about going from 25-35hp and he helped me out a lot with my questions etc.... so im gonna make a PDF file with all the part numbers and other information that is relevant to these conversions for motors from mid 70's through mid 90's..... I will post on here when im done and will send it to anyone who wants it... not that johnny25 is being bothered(I think he likes talking about it) but this way people can have a cross reference chart.....

 

[alfaguy]

Took my 25/30 to the river and it ran really well except for a severe misfire at 2200 rpm. It is fine at 2000 and 2500 but totally breaks up at 2200. It has a new power pack, coils and plugs (but not wires).

Any suggestions on how to cure it.

Also I could not get above 4500 rpm with a 10-13 prop. I may need more tuning.

 

[Siguz]

Took my 25/30 to the river and it ran really well except for a severe misfire at 2200 rpm. It is fine at 2000 and 2500 but totally breaks up at 2200. It has a new power pack, coils and plugs (but not wires).

Any suggestions on how to cure it.

Also I could not get above 4500 rpm with a 10-13 prop. I may need more tuning.

Sounds to me like it wants more fuel.

 

[alfaguy]

I think it needs fuel during the transition from low to high speed. I will flush the transfer holes again to see if something got in there after I rebuilt the carb. The main jet is 0.067 so it should be getting plenty of fuel at high speed.
I am having trouble with the Faria tach above 3500 rpm so I am not sure about my max rpm it is turning.

 

[Siguz]

I had a similar issue after I did my mod. All I ended up doing is rebuilding the fuel pump and flushing lines. Everything was fine after that. I don't know if that was the cause but it only took 15min.

 

[Edstrossner]

Great info!

But I am looking to go farther in the 25 to 30 hp mod.
I recently bought a 16 foot Lowe with a 2000 25 hp Johnson, 2 cylinder cross flow.
The stock carb has a .875 Venturi with a 1.250 throttle bore. Wot is 5500rpm at 24mph gps.
I had an old 70 hp carb with a 1.187 Venturi and a 1.375 throttle bore. It bolted up to the intake although the linkage was slightly out of line. The 70 hp carb has a longer throttle shaft. The motor fired up easily with the big carb and idled a bit better than the stock carb. At wot there was only a slight increase, 5800rpm at 25 mph gps. It appears that the motor cannot take advantage of the bigger carb.
I have modified many larger omc crossflows in the past with good success but I need some measurements from a 30hp block If anyone has one.
I seen one post than indicated that the 30 hp ports are oval. How many ports on the intake side? How many on the exhaust side? What is the deck to the top of the ports measurement? What is the size of the passage way from the crankcase to the transfer ports?
Any info would be greatly appreciated!

 

[Siguz]

Great info!

But I am looking to go farther in the 25 to 30 hp mod.
I recently bought a 16 foot Lowe with a 2000 25 hp Johnson, 2 cylinder cross flow.
The stock carb has a .875 Venturi with a 1.250 throttle bore. Wot is 5500rpm at 24mph gps.
I had an old 70 hp carb with a 1.187 Venturi and a 1.375 throttle bore. It bolted up to the intake although the linkage was slightly out of line. The 70 hp carb has a longer throttle shaft. The motor fired up easily with the big carb and idled a bit better than the stock carb. At wot there was only a slight increase, 5800rpm at 25 mph gps. It appears that the motor cannot take advantage of the bigger carb.
I have modified many larger omc crossflows in the past with good success but I need some measurements from a 30hp block If anyone has one.
I seen one post than indicated that the 30 hp ports are oval. How many ports on the intake side? How many on the exhaust side? What is the deck to the top of the ports measurement? What is the size of the passage way from the crankcase to the transfer ports?
Any info would be greatly appreciated!

The best info I can offer is the parts catalogue from Evinrude.
I have been using this to compare parts and fitment.

https://shop2.evinrude.com/Index.aspx?s1=skj55hhutt3n3m40hn0qm2jom5&catalog_id=5&siteid=1

 

[Edstrossner]

The parts catalogue is a good start but I'll be doing some machine work to this block and it helps to know what OMC did in their port placement to get 30hp.
I'll be squaring the ports with a 1/4" end mill and possibly opening up the passage way from the crankcase to the transfer ports. It's easy work with a Bridgeport mill.
I also had a friend of mine point me towards the Chrysler 150hp crossflow. They put in a boost port by drilling a large hole in the piston and the transfer port passage. Interesting stuff.

 

[Lil' Blue Rude]

I think I'd use at least a 3/8 end mill. Using a 1/4 end mill will give you small radius at the corner of the ports and I think that would be pretty hard on the rings. Seems like some of the inline 6 mercs use a boost port. Haven't heard of many good results from porting on these motors. People talked like they made a little more power, a lot more noise and they drank a lot more gas. I'm sure that was porting beyond 30hp specs though. You might look at scream and fly for what they call the "nine pages" it's mods for the om v6 crossflow motors and I know some people have used the same basics steps on the omc v4's.

 

[Edstrossner]

I think it was a 3/8" end mill that I used on the V-4's. It was a long reach to square off the exhaust ports and they did not make an end mill long enough in 1/4".
I do not want to change the port timing on my 25hp. If I did change it, It would be to match up to the 30hp specs. It seems that the 25hp block cannot take advantage of the larger carbs. There must be some flow restrictions that can be opened up.
Has anyone checked compression on these motors? What is good compression on these? The V-4's and 3 cylinders that I have built would be in the 140 to 150 psi range. After porting I will rework the head to bring the compression up.