I'm getting a mid 70's Evinrude 15HP outboard soon. I had another 15HP Evinrude (about the same year) a while ago on my jon boat and just wasn't quite enough for it. I was wondering if it's as "simple" as a carb upgrade to bump it up to a 20? I've heard of people doing this with a 9.9 up to a 15 before, was just curious if its the same from a 15 to a 20 or is there more involved.

Im pretty sure the 9.9 to 15 hp is folklore... there are more differences in those motors than a carb.  Different reeds etc.... most likely if you did this you would end up with a poorly mixed 9.9 and nothing else. I would think the same for your case but someone may know better.

I know since buy my new boat with a 2004 30hp tohatsu 4-stroke I have read that the 25hp is almost the exact same motor except for something in the carb and the ecm or ecu. Changing those two things make it a 30hp but those two changes are like $1100. Yours could be cheaper and easier or the cubic inch of your engine may be the same as the 9.9 meaning you can't up the hp anymore.

If you could I would be curious what the implications were for wear on the motor. Not sure if this is even remotely the same, I remember some dude in school buying all sorts of crap to up the HP on his car's engine. It worked great, except he blew the engine up real quick, the entire system was just not made to run like that...

<warning: the above message is NOT from someone who works with these motors on a regular basis....>

Thanks for the responses everyone! Obviously, I'm only interested in this IF the engine itself is the same size and its only a different carbuator or other small modifications that increase the horsepower. I'm not looking to do anything that could shorten the life of the motor, I'm just checking to see if it is an option worth persuing. Thanks again for the replies so far!

I'm getting a mid 70's Evinrude 15HP outboard soon. I had another 15HP Evinrude (about the same year) a while ago on my jon boat and just wasn't quite enough for it. I was wondering if it's as "simple" as a carb upgrade to bump it up to a 20? I've heard of people doing this with a 9.9 up to a 15 before, was just curious if its the same from a 15 to a 20 or is there more involved.

You might want to post a forum thread at www.screamandfly.com

Those guys hop up motors for racing. Im sure someone there will have the answer for you.

The 20 is a totally different powerhead, it's the same as the 25, so NO, you can't easily make your 15 at twenty.

The 9.9 and the 15 are the same powerheads so it was fairly easy to convert the 9.9 to a 15 by simply changing the intake and carburator

By the way, this is the same reply I would have given you on S&F.

It is very common for manufactors to limit horse power by simply installing a smaller/larger carburator or by using different programming in the ECU on the new motors.  For instance, for many years the only difference a 200 hp and 225 hp, three litre OMC has been the carbs.   On the 3.3 DFI motors, most of those are just programmed to make the hp the want.

Well, that shoots that thought in the foot!  ;D

Thanks for the confirmation Way2slow!