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johnson 25hp change upto 30hp and 35hp by changing venture on carby

K

kennykoala

New member
g,day all just got back from a weeks fishing at Mallacoota victoria,great caught salmon tailor kuda mackrel trevaly pinkies,Mallacoota was quiet but the navy jetty was hot.back to advice needed my neighbour as a 25hp jon same as mine he bought a 30hp and he changed the carb onto is 25hp and he rekcons this can be done and works ok.? what are your thoughts.is 30hp he just bought was in pieces so thats why he put the carby on his 25hp.ty all.
 
If both motors are near the same year it should work fine , in order to up HP you might have too change the LU as a 25 and 30 usualy have diffrent gearing that makes the HP, and not the carb, as both blocks are the same for most all those motors. Check out the specs for both motors in order to find out for sure.

Great Fishing an Be Safe

Oldman570
 
To get the HP other things need to be done also. Checkout the replies to Thread "
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carbsStarted by shawnfish‎, 12-18-2011 07:42 AM" farther down this list. It can be done without to much trouble. The only big cost and trouble is finding a motor, with most everything in working order, and a bad power head.
Merry Christmas ,

Oldman570
 
oldman570 said:
If both motors are near the same year it should work fine , in order to up HP you might have too change the LU as a 25 and 30 usualy have diffrent gearing that makes the HP, and not the carb, as both blocks are the same for most all those motors. Check out the specs for both motors in order to find out for sure.

Great Fishing an Be Safe

Oldman570
Click to expand...

first of all there more to it than"if both motors are near the same year" it should be ok. ........and second the gears have absoluteley NOTHING to do with HP gain or loss. third. it is absoluteley the carburetor that makes the difference in HP. the more oxegen/fuel you give the motor the more power you get. the carbs are the same except for the high speed orfices (35 is bigger) and the venturi's, the 25 has a restricter inside it making it not able to feed the engine as much oxegen/fuel as the 35carb therefore making the 25hp carb only capable of producing 25hp from the same 521cc powerhead that makes 35hp. also you cant just change the lowers as they have different exhaust housings with the same tubes. and in this case we have no clue what year motors were talking about. most of the 20/25/30 and 35's are pretty much the same motors but depending on the years you may have different intakes with a restrictor plate that you need to very carefully file away wich ive seen done and works but I wouldn't do it. and if one of the motors was say a 25 from 1985 and you were gonna put a 30carb off a1984 model you get no gain due to the change from flywheel hp rating and the prop hp rating.. if you do your homework cross referencing part #'s you can gain hp by changing carbs, I did it to my 1980 25 by putting the 35 carb on it and it works. and on a side note even with the different lowers,gears and gear ratios on the 25's and 35's I was told by a few OB guys that the 25 lower unit and gears will handle the extra hp no problem due to the fact that OMC basically over engineered the 40 and under hp and pretty much made them indestructible and the proof is in the thousands of them that are still running after 40 or 50 years.......
 
Besides changing the orfices (or drilling yours out) you also need to get a template and over size the manifold opening so the engine can also draw more air as well as fuel from the carb. Be sure if you decide to dremel out the manifold you remove it from the engine first.
 
daninski said:
Besides changing the orfices (or drilling yours out) you also need to get a template and over size the manifold opening so the engine can also draw more air as well as fuel from the carb. Be sure if you decide to dremel out the manifold you remove it from the engine first.
Click to expand...

on every one besides a few yrs I looked into you do not have to dremel anything. all the manifold/carbs have the same ID and the few that have a restricter plate that is on the manifold all you have to do is get a different yr manifold without the restricter instead of using a dremel although ive seen it done but I wouldn't do it. the ones I seen with the restricter were all 20hp, I looked up most years but not all.
 
kimcrwbr1 said:
All we can do is speculate as we do not know what we are looking at. You need to look at all the factors lower unit being the least important. A split case gearbox will be able to handle the added ponies easily if maintained properly. Venturi, intake port size and exhaust port size are the most important things to look at. Even if the displacement is there compare intake manifold and exhaust inner tube part numbers if they are the same then the larger venturi and jets will work just fine. You can usually tell by WOT rpms. A detuned 35hp down to 30 or 25 will have lower WOT rpm range. A model number will determine what options are available if any.
Click to expand...

X2 almost every 25-35 I looked up have different exhaust housings but the inner tubes were the same
 
Hey folk. New to posting, but have greatly benefitted from the information exchanges in this forum so would like to say thank you all for contributing and very thoroughly and digestible more times than not. Just wanted to share my experience in this endeavor. I’m currently working on a ‘73 rude 25hp 2-stroke with an electric start up-fitting. I have the motor running and reaching decent high rpm while in neutral but under-load it won’t wind past 1/2 max. I’m pushing a lightweight 14’ aluminum flat bottom but it’s normally weighed down by trot lines, anchors, my buddy, my dog and, I think, myself. Estimated weight? I dunno, touching on 800 lbs max possibly. So I had a 198?Johnson 25hp motor I bought for parts and I noticed the carb in it was substantially larger in terms of volume and carb air intake/ throttle body opening size etc. so I cleaned it and blew it out and didn’t see need to replace the gasket as it seemed to had been recently replaced. I attached photos comparing the old to newer carbs. The holes for the studs I thought were exactly the same but upon removing and holding intake ends face to face realized a different if maybe 1/8”, the newer carb having slightly wider mounting holes. I used a die grinder to make a bevel on the hole going into my manifold that is the size of new carb opening down to maybe an eighth inch bigger diameter than what it originally was, then sanded smooth as I could. I will have to trim the ID of the gasket to the new hole size or I believe the whole intended purpose will be for not and I had to ream the newer carbs mounting holes in the eight inch or so to fit onto the studs. I am aware that a new larger 30 or 35hp manifold would be ideal to put on but I’m reserving that option in the likelihood that this doesn’t work out. Thoughts, criticism or suggestions are greatly appreciated.
 
Hi, you have to size photos under 1000k. Be nice to start a new thread too. This is over 10 years old.
 
Ok I’m gonna start new thread Tim standby sir. I’ll name it “1st attempt on conversion on 1973 evinrude 25hp to 30/35hp using carb and manifold mods”
 
bigger carb on a small motor doesnt always work well heres an example if you put a 500cfm circle track holley on a IH 345 it will run like garbage and off idle will be a nightmare to to get corrected with powervalve and jetting changes etc because the larger bore of the carb doesnt allow the weak vacuum signal to pull fuel from the bowl in a timely manner and you get a off idle stumble. that said a little larger carb on a motor will likely not have these issues untill you make several steps in size
 
shawnfish said:
first of all there more to it than"if both motors are near the same year" it should be ok. ........and second the gears have absoluteley NOTHING to do with HP gain or loss. third. it is absoluteley the carburetor that makes the difference in HP. the more oxegen/fuel you give the motor the more power you get. the carbs are the same except for the high speed orfices (35 is bigger) and the venturi's, the 25 has a restricter inside it making it not able to feed the engine as much oxegen/fuel as the 35carb therefore making the 25hp carb only capable of producing 25hp from the same 521cc powerhead that makes 35hp. also you cant just change the lowers as they have different exhaust housings with the same tubes. and in this case we have no clue what year motors were talking about. most of the 20/25/30 and 35's are pretty much the same motors but depending on the years you may have different intakes with a restrictor plate that you need to very carefully file away wich ive seen done and works but I wouldn't do it. and if one of the motors was say a 25 from 1985 and you were gonna put a 30carb off a1984 model you get no gain due to the change from flywheel hp rating and the prop hp rating.. if you do your homework cross referencing part #'s you can gain hp by changing carbs, I did it to my 1980 25 by putting the 35 carb on it and it works. and on a side note even with the different lowers,gears and gear ratios on the 25's and 35's I was told by a few OB guys that the 25 lower unit and gears will handle the extra hp no problem due to the fact that OMC basically over engineered the 40 and under hp and pretty much made them indestructible and the proof is in the thousands of them that are still running after 40 or 50 years.......
Click to expand...
I have a 1980 25hp Johnson and wanting to throw the 35hp carbs on it. I have a late 70s 35hp. Will the carb and intake off that match up on my 25hp?
 
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