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25hp to 35hp 2004

J

Jeff Murray

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I'm sure you guys have had this question alot of times, don't give me a hard time, I searched all I could and found that maybe I could swap out the Carb. But I'd like to know what EXACTLY to do to make my 25hp johnson, into a 35hp? Thanks.
 
If it can't be made into 35hp, I basically would just like to know how to increase the power on the motor. I appreciate any answer/help you can give me...
 
In the U.S. it's illegal to alter the horsepower, but up here in Canada there is no regulation. So from time to time there is the odd fellow that I can't talk out of trying to increase the horsepower so I do, do the work (after all, at the end of the day my job entails making money).

There are some models that you just can't "hop up" any without going the "franken-outboard" route. In the smaller outboards 15's and 25 horses tend to be one (often they can be detuned to a 9.9/20, but not up to say a 20 or a 30), but if you do post your model we can certainly take a look. If the motor is already at the (max) for the particular block there is really no where further to go and maintain a stock engine.

On the models that can be modified it's sometimes as easy as removing a restrictor plate or as complex as replacing the carb, reed cage and exhaust. In all cases the owners of these "improved" motors are almost always disappointed because they notice "no difference" (except the few to many less dollars in their wallet). But there is the "mental" improvement - they know they have more horses even if the GPS isn't confirming it with greater amounts of speed.

Rule of thumb: you require a 50% increase in horsepower to see a 15% increase in speed when talking about a marine engine - going from 25 to 30 horses (20%) would be a 6% increase. So if you current top speed was say 25 mph, your new top speed would be a touch over 26 mph - yipeee...

I'm willing to bet that you could bring your 25 horse to me and I could do nothing more than replace the sparkplugs and de-carb the powerhead and tell you that I hopped it up to 32 horses - charge you 500 bucks in the process and send you on your way. You could even run head to head with your buddy at the dock whose running a 30 horse and you would be impressed how well you kept up and we both would be happy - but like increasing the horsepower of a U.S. recreational outboard, that would be illegal as well :)
 
The model number is J25TELSOM Which means it's a 2005, not an 04. Thanks for your info, just wondering if there is anything that can be done that won't break the bank. But like you said, I might not even be able to tell a difference.
 
You wouldn't. There is nothing you can easily "bolt on" to a stock outboard that will produce any "noticable" performance improvement (read top speed). Since a boat is in essence pushing tons of water out the way until it climbs on top and "planes", it takes gobs of extra horses to significantly increase top speed.

You will see ads for "fins", boyseen reeds, fuel additives, high power coils, fancy props etc that claim "increased speed and fuel savings" and they are technically true, but you are usually talking about 1/100ths of a mph or gph in improvement.

If your top speed with a 25 horse on a small utility is say 25 mph, moving up to a 40 horse would in theory move you up to 29 mph max, but since a 40 horse will tip the scales at about 200 pounds and the 25 was only maybe 125, you lose 2 of those miles per hour because now you are pushing more weight (on a 14/15' aluminum 75 more pounds could be equal to 10% more total weight). So your net gain may be less than 2 mph at top end, although it would have significantly more "guts" at the lower end because of the increased displacement - but it would burn more gas all the time (a 40 would burn about 40% more gas than the 25 "all the time").

So it really is diminishing returns since most boats could not safely hang enough horsepower off the back to make that real difference you would be looking for.

If your current boat is fitted with 80% of the max horsepower recommendation listed on the plate, then your boat will perform at the max potential it was designed for (each hull has a maximum speed that it can achieve before it becomes unstable - that listing on the plate IS just a recommendation, but if you exceed it, the boat could handle very dangerously).

Running fresh, stablized gas each time out - changing the sparkplugs once a year (whether they need it or not) and changing the impeller every second season will do more to improve the performance of the average motor than all the fancy bolt on performance improvements being sold at the local boat shop...
 
What I'd really like is "Guts" more than top speed. Where I do most of my fishing with this particular motor, there are always some logs that I have to traverse but it is what it is. I appreciate the help.
 
Not sure what size prop you are using now but if you change from say a 15 to 13 you may get the results you are looking for. Note that this will also raise rpm at top speed and can cause engine to rev. too high causing engine failure. So be carefull not to go too small.
 
galamb said:
In the U.S. it's illegal to alter the horsepower, but up here in Canada there is no regulation. So from time to time there is the odd fellow that I can't talk out of trying to increase the horsepower so I do, do the work (after all, at the end of the day my job entails making money).
Click to expand...

Being an older American 55+ I would like to know what the reference is for that?? I thing that is a rumor. I have never heard of that being illegal. The only thing we do have are racing regulations that limit cubic inches.

Now the outboard manufacturer's make various models that have been modified and de-tuned to allow people to match the Maximum Horsepower Allowed by the USCG regulations and the BIA Standards.
 
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