Changing Johnson 25 hp to a 35 hp??

Discussion in 'Outboard Maintenance' started by blondmonkey777, Dec 1, 2012.

  1. Have read a few things on the internet that says that the block for them are the same for 20hp,25,hp,30hp and 35 hp what all would be involved with changing it? I was thinking just the carb
  2. alain_vallejo

    alain_vallejo Screw work!!! I'm going fishing!!!

    I would look at the parts diagram, all the numbers and compared all of it. I did it to a yamaha 9.9 and made it a 15 all by changing the reed restrictor plates.

  3. Wheres the easiest place to find one of these?
  4. Dadvocate

    Dadvocate If it ain't broke tweak it a little....

    343 they have good diagrams and good service
  5. firecat1981

    firecat1981 BBA Counselor

  6. Creek Runner

    Creek Runner Well-Known Member

    What year is your motor? Pre 95 only thing that needs to be changed is the Carb & Intake. In 96 and later it was an entirely different block assembly. Just an FYI it's not going to make much a difference I have done probably 50 conversions and your talking .05 shaved off your hole shot and maybe 1-2 mph increase on top speed, although it does tend to idle better.
  7. Its a 2005 what would have to be on that year model to upgrade hp and how much hp would you really gain? thanks for all the info
  8. Creek Runner

    Creek Runner Well-Known Member

    It's an entirely different block assembly; you would be better off buying a 35hp then trying to convert that one. I’m actually friends and worked for the engineer who designed the OMC 35hp. And as I said in my previous post you’re not going to realize much gain, the guys who say otherwise are also the guys who think that a K&N filter makes their car/truck faster.

    Look for a 40hp if you want more performance.
  9. Im using the 25 hp because I need a light weight motor im pretty sure that making a 2005 johnson 25 to a 30 is what just a different carb?
  10. Creek Runner

    Creek Runner Well-Known Member

    Yes the 25hp to the 30hp is only the carb, the 35hp is when the block assembly changes.
  11. el9surf

    el9surf Well-Known Member

    Just for reference my 25hp at wot starts to overpower the shadowcast anyways. Not sure more hp is going to help things.
  12. Dillusion

    Dillusion devilray snob

    Did you have the built in bow fuel tank 8gal? I rode on skinny dippin's shadowcast during the ankona weekend and we were fine with the 25 at WOT
  13. No I dont have the built in fuel tank but I wouldnt mind the extra 5 hp lol