how to soup up my '73 25hp johnson

[jgouras]

well im looking to get a little more power out of my 25horse. any ideas? i put a jet pump on and i need a little more to push my boat around. i have read somewhere about changing carbs with a 35 horse or something like that. any help would be great.

 

[kbait]

Re: how to soup up my '73 25hp johnson

Search this forum. I remember reading a lengthy discussion on the 25-30-35 tweaking.. I've heard that jet pumps do cost on the speed end.

 

[Willyclay]

Re: how to soup up my '73 25hp johnson

Here's a link to a pretty thorough discussion of your question:

http://forums.iboats.com/showthread.php?t=274566

You'll have to decide what will work for your motor. Good luck!

 

[F_R]

Re: how to soup up my '73 25hp johnson

What do you expect already? Your '73 25hp is aready souped up!!!!! The 22 cubic inch 25 started life as an 18hp. You aren't going to get any more out of it except by getting a bigger motor. Assuming you want to keep the jet drive.

 

[14ftgrumman]

Re: how to soup up my '73 25hp johnson

There'ss not a lot that can be done that wasn't done by OMC when they made the 18 into a 25.

Easiest and less frustrating improvement would be to invest in a 30-40hp motor.

 

[tashasdaddy]

Re: how to soup up my '73 25hp johnson

changing to jet drive, you lose 30% of the output horse power. go back to the prop.

 

[Chinewalker]

Re: how to soup up my '73 25hp johnson

Your 25 shares very little with the later 25/35 platform. Yours is 22 cubic inches, while the later ones are 33s. As FR noted, your motor is already about as tweaked as it can get as you're already getting nearly 50% more horsepower out of it than when it was originally designed...

 

[mikesea]

Re: how to soup up my '73 25hp johnson

see if you can get boyseen reeds for it,they apparently help

 

[1946Zephyr]

Re: how to soup up my '73 25hp johnson

Yep, like the guys said...the 25 is an upgraded 18 and there isn't really anymore you can do with it. The 18-20 hp got their rating at 4500 rpms, where the 25hp has it's rating at 5500 rpms. Simply speaking, the 25 is just a higher revving 18 and I think that comes from larger carburation venturis. I can't think of anything else it could be. You could possibly rebuild the engine and install high performance pistons in it and possibly port it out. I doubt if there ever was a dual carb manifold built for the engines or turbo chargers.

 

[kenmyfam]

Re: how to soup up my '73 25hp johnson

Best way to "soup up" a 25hp outboard is to sell it and buy a bigger one.
Just my personal opinion.

 

[Howard Sterndrive]

Re: how to soup up my '73 25hp johnson

the existence of a rare omc factory 12P Stainless prop for the 18-25 gearcase suggests there is some "official" way to get more ponies, but the internet sure ain't talkin'

I hope to stumble across something on this someday in the piles of E/J books and notes I have......

neither of my 25's will get that 12P SS prop above 4600rpm on my 11' inflatable and I can't imagine these motors on less boat than an 11' blowup

The options are:
1. make more power
2. lose weight

so I wanna mod the 25

 

[Faztbullet]

Re: how to soup up my '73 25hp johnson

That a perfect prop to run on a little stock A or J class boat!!!

 

[Rick.]

Re: how to soup up my '73 25hp johnson

Give it a paint job with flames? Rick.

 

[moonfish]

Re: how to soup up my '73 25hp johnson

That's awesome. You could clamp it on some jetski guy's trailer tire and the cops would chase him down! Or send them after some drunken hotrodder! The possibilities are endless.

 

[F_R]

Re: how to soup up my '73 25hp johnson

Yep, like the guys said...the 25 is an upgraded 18 and there isn't really anymore you can do with it. The 18-20 hp got their rating at 4500 rpms, where the 25hp has it's rating at 5500 rpms. Simply speaking, the 25 is just a higher revving 18 and I think that comes from larger carburation venturis. I can't think of anything else it could be. You could possibly rebuild the engine and install high performance pistons in it and possibly port it out. I doubt if there ever was a dual carb manifold built for the engines or turbo chargers.

Simply not just a matter of larger carburetor. The 25 has different pistons and cylinder head. And a different (balanced) crankshaft to handle the revs.

The 25 revs higher because it has more horsepower to turn the prop faster.

And that brings up another point. A 25 won't be much faster than an 18 unless the boat and prop allow it to go faster. This all gets back to why the jet drive is so slow. Too much inefficiency and wasted horsepower in the jet. You simply can't put a race engine in a dump truck and expect to win at Daytona.

 

[kenmyfam]

Re: how to soup up my '73 25hp johnson

You simply can't put a race engine in a dump truck and expect to win at Daytona.

Well bang goes my next project then !!!!!!!

 

[jgouras]

Re: how to soup up my '73 25hp johnson

thanks for all the responses. actually i was not looking for all the ways you couldn't do it, but all the ways you could I need the jet drive for where I fish so I can not just go back to the prop, which I do when I don't need the jet drive. Looks like a bigger engine for me then.

 

[1946Zephyr]

Re: how to soup up my '73 25hp johnson

Simply not just a matter of larger carburetor. The 25 has different pistons and cylinder head. And a different (balanced) crankshaft to handle the revs.

The 25 revs higher because it has more horsepower to turn the prop faster.

And that brings up another point. A 25 won't be much faster than an 18 unless the boat and prop allow it to go faster. This all gets back to why the jet drive is so slow. Too much inefficiency and wasted horsepower in the jet. You simply can't put a race engine in a dump truck and expect to win at Daytona.

Oh, I was wondering what all they did. I know they're 22 cubes, like the 18, but where did the increase really come from? Now I know. So, a guy could actually rebuild an 18 or 20 and install the components then?

 

[F_R]

Re: how to soup up my '73 25hp johnson

Oh, I was wondering what all they did. I know they're 22 cubes, like the 18, but where did the increase really come from? Now I know. So, a guy could actually rebuild an 18 or 20 and install the components then?

Sure, if you buy enough parts. Actually, I have already done it. I gained about 2MPH. I'd say it wasn't worth it.

EDIT: But then I replaced the motor with an '84 35hp and it made a huge difference. Now it runs like the proverbial Striped Ape.

 

[1946Zephyr]

Re: how to soup up my '73 25hp johnson

I think it would be cool to do this with a '57 18 Fastwin