1991 25hp short shaft mods

[ArDuckhntr]

Hello to all,

New to the forum, but have been lurking in the background for some time reading post. I own a 91 25HP Johnson short shaft tiller that I would like to supe up a little. I'm sorry I do not have the exact model number at this time.

Currently I'm running a 12p prop, and I'm able to pull around 4950rpm WOT. I would like to get this little motor to sing even more if possible. I currently have a tiny tach installed and that is it for mods. This motor is used mainly to duck hunt and I would like to carry a load better, as well as get as much speed out of it as possible.

Thx for the help...

 

[tashasdaddy]

Re: 1991 25hp short shaft mods

balancing your load in the boat, is of utmost importance. if you go up in prop pitch, you may get a little faster wot, but you will loose bottom end getting on plane.

 

[ArkDuckhntr]

Re: 1991 25hp short shaft mods

Understood about the load; I'm from the Mud Motor days, both long and short tail so I know about balance. Also, I've got a 13p prop that I use as well. I saw a post about some guys taking 20hp units and throwing on a 35hp carb w/light mod work and getting good numbers. Can that be done on my 91 25hp model? I run ducks in Arkansas obv. and they have limited hp numbers on WMA's. Guys here normally run the 25hp Suzuki on lightweight hulls to get to high end numbers. I know I won't touch those rigs, but would love to get more out of the same little motor if possible with just a few mods.

 

[ArkDuckhntr]

Re: 1991 25hp short shaft mods

I've not seen any responses to my post with regards to possible carb change. Is it possible to swap the carb directly on a 90-91 25hp Johnson with a like year 35hp carb? According to a post I found it may be possible, but never really spelled out the specifics on make/model numbers.

 

[Brokeback Johnson]

Re: 1991 25hp short shaft mods

The carb from a 35 should work but you will also need to change the tuner pipe to accomodate the extra exhaust flow.......I believe that would give you the full 35hp out of that powerhead.

 

[ArkDuckhntr]

Re: 1991 25hp short shaft mods

change the tuner pipe to accomodate the extra exhaust flow.

I've worked on motors before, and even modified a few Mud Motors in my day; how much time are we talking to make those mods? Also, what if any setting changes will need to be made to get things in tune for the increased flow both on the intake side as well as the exhaust side?

 

[wilde1j]

Re: 1991 25hp short shaft mods

You'll be way ahead just buying a bigger motor. Just changing the carb will result in more gas use, but almost no power change, w/o a bunch of other changes.

 

[ArkDuckhntr]

Re: 1991 25hp short shaft mods

My state has a HP limit on motors run on WMA's; I'm trying to keep the stock look of my 25HP, but have it modded to run faster like a 30+HP motor.