Want to make 18 hp evinrude into 25 hp

[BassAckwards80]

I have a 1968 18802m 18hp evinrude. I also have a 1978 25 hp 25r78c johnson that's a parts motor. I prefer the evinrude as it's just lighter and nicer looking.

I've been told that all I need to do to make it 25 hp is to change carbs. What all do I need and need to do to do this?

 

[nwcove]

Re: Want to make 18 hp evinrude into 25 hp

welcome to iboats bass! i dont think you can accomplish what you want to do without some serious and costly mods. just swapping a 25 carb ( will it even bolt up?) to the 18 block will most likely rob hp and waste fuel. id suggest tuning the 18 to make it run strong and enjoy it for what it is.

 

[HighTrim]

Re: Want to make 18 hp evinrude into 25 hp

Sounds good in theory though doesn't it!

 

[BassAckwards80]

Re: Want to make 18 hp evinrude into 25 hp

I actually have 2 of the evinrude motors. They are phenomenal motors! The motor I would be tinkering with has quite a bit of repairs needed (costly). So I wanted to experiment with the carbs. I haven't tried even bolting the carb up but figured it would be different. Any advice or ideas?

 

[F_R]

Re: Want to make 18 hp evinrude into 25 hp

The major differences between the 18 and 25 are:
Carburetor
Pistons
Cylinder head
Crankshaft

That old rumor about just changing the carburetor is BS. If you don't change everything, you will not get what you are looking for. May even be detrimental.

 

[nwcove]

Re: Want to make 18 hp evinrude into 25 hp

my advice would be to use the best powerhead you have of the two, tune it up PROPERLY and run it. those 18 hp motors are the best power to weight ones ever built by any mfg. (jmo). other than that messing with the exhaust porting can give you some gains, BUT....it can also and in many cases make a perfectly good powerhead a piece of scrap. save your parts motor for parts ( its not like there is an endless supply of "hard" parts!), and go boating, or buy a good 25hp and sell the others to help fund the purchase. jmo

 

[steelespike]

Re: Want to make 18 hp evinrude into 25 hp

Your best bet is to find a pre 76 25 hp Based on the 22 cube power head. Your 78 25 is a 31.8 cube power head.

 

[steelespike]

Re: Want to make 18 hp evinrude into 25 hp

Your best bet is to find a pre 76 25 hp Based on the 22 cube power head.87lbs Your 78 25 is a 31.8 cube power head. 108 lbs.

 

[BassAckwards80]

Re: Want to make 18 hp evinrude into 25 hp

Thx for the advice guys.

Are there any good modifications that I can do that are worth while?

 

[nwcove]

Re: Want to make 18 hp evinrude into 25 hp

Thx for the advice guys.

Are there any good modifications that I can do that are worth while?

the engineers that designed them did all the tinkering to make those motors work right from idle to wide open, anything you do will cause issues one way or another. tune the motor up, run some seafoam through it, and enjoy a motor that will probably still outlast one thats been built in 2014!!!jmo