# anyway to make a 20hp into a 25hp merc?



## jimbarn1961 (Aug 2, 2009)

Just bought new skiff, maiden voyage runs great. Just need more speed SCOTTY . I was reading owners manuel for 2002 mecury 20/25hp everything is identical bore, stroke, displacement, plugs, gear ratio, only difference noted was 20hp 4500-5500wot and 25hp 5000-6000wot. Can I find the other 5 ponies and how? carbs, jetting? thnx for help guys.


----------



## Brett (Jul 16, 2008)

The jet is different, and the carb throat diameter.
Looks like you'd have to swap carbs.

http://www.crowleymarine.com/mercury_parts/3236/110.cfm


----------



## Brett (Jul 16, 2008)

I think it's a different carb body also, check the part numbers in the link,
larger diameter throat to allow for more air flow.
More air, more fuel means more horsepower and higher rpms.


----------



## jimbarn1961 (Aug 2, 2009)

Ok, so now I know need a new carb. to find those 5hp or 25%increase in power. 
Anyone got a spare 2002 mercury 25hp Carb, #817405t23 reasonably priced and functional? PM me.


----------



## Gramps (Nov 1, 2007)

Don't dump money into it yet Jim!!  Check your RPM's, see where the cav plate is in relation to the bottom of the boat. What pin is the trim setting? How much weight was in the boat and how was it distributed?

PS - Remember 16 year old kids will get in trouble with speed! ;D


----------



## jimbarn1961 (Aug 2, 2009)

Scott, 

Cav plate when measured off bottom of boat is 1.5" below bottom of boat when measureing off a straight board held flat to the bottom. Also am ordering a tiny tach this week. motor is trimmed up in 2nd of five holes tilting up from transom. Also motor is already lifted up one inch above top of transom already. So do not think I can raise another 2.5" without jackplate. I guess need to get motor corrected first b4 looking for more hp?


----------



## rkmurphy (Nov 2, 2008)

That motor runs beautifully level with the bottom of the boat. Never had it higher so I can't say anything about that. Get a TomC or Bob's manual JP. Shouldn't cost too much and will be adjustable.


----------



## jimbarn1961 (Aug 2, 2009)

RK, 

Does that knowledge make you the previous owner?


----------



## rkmurphy (Nov 2, 2008)

I did happen to own a 25 Merc 2 stroke! As still pictured in my signature...I need to change that.

Your motor is identical. Just different carb. Badass motor...probably not a 2 stroke out there that can run as good as the Merc 25.

Tune her up, replace that carb, and enjoy a bulletproof motor. Oh...and don't take it to a dumb-ass mechanic...I made that mistake and it ruined any good feeling I could have possibly had about that motor.

Enjoy!


----------



## Brett (Jul 16, 2008)

marine salvage shop might save you a few bucks

http://www.marinepartsoutlet.com/index.htm


----------



## TomFL (Oct 2, 2007)

> marine salvage shop might save you a few bucks
> 
> http://www.marinepartsoutlet.com/index.htm


These are great people to work with, if you call the Stuart location ask for Bob or Dee. I've been in there A LOT lately to pick up misc stuff while rigging the boat. 

Give them a shot, they may have what you're looking for. 

-T


----------



## jimbarn1961 (Aug 2, 2009)

Will check it out, it is in my area within an hour drive. But outboard mounting height issues might need to be addressed first. then tach, carb and prop it out or some such order as that.

My name is Jim I am a BOAT ADDICT ;D


----------



## DuckNut (Apr 3, 2009)

Are you guys saying you can make a Chevette a Corvette just by changing the letters?

There has to be more to it than that.  I just can not believe it is that easy.

Question: If I bolt on a carb from a Corvette onto a Chevette will I get Corvette performance?

Answer:  No.

Power comes from a blend of energy in AND energy out.  Although I am not an expert at this I do know a little bit.  If you can get more gas into the engine you have to get it out as well.  I firmly believe that you would also have to make some improvements to the exhaust ports and on a 2 stroker that is just as important if not more important than intake.  If on the exhaust stroke the ports only let "X" amount out, then on the intake stroke the system will only let that same amount of "X" in.   

I may be completely off my rocker with my thought pattern, but I think this would be a great question for Capt. Jan to ask his contacts from where he got the motors for the shootout.


----------



## Brett (Jul 16, 2008)

http://www.crowleymarine.com/mercury_parts/3236/130.cfm


same exhaust manifold DN, you made a good point so I looked it up.
Apparently the smaller bore carb acts like a restrictor plate in NASCAR,
in that it prevents total use of available designed horsepower.
Even the reed block has the same part number.
I'll bet the timing and throttle linkage needs to be adjusted to fit the higher rpms.


----------



## mark_gardner (Mar 18, 2009)

i have a buddy in boynton bch with a 25 merc 2 smoke looooong  shaft   ;D, i think its a 1999 model?? that he's looking to sell for parts, one of the pistons burned up in it according to his mech. but everything else is supposed to be good. it might have everything you need to make your 20 in to a 25 let me  know if your interested and i'll give you his contact number


----------



## Gramps (Nov 1, 2007)

> Are you guys saying you can make a Chevette a Corvette just by changing the letters?
> 
> There has to be more to it than that.  I just can not believe it is that easy.
> 
> ...


You're pretty much on the ball. Over the years it seems outboards manufacturers have setup one production run for two or even three different HP motors. I know my old 9.9 Evinrude was the same as a 15, just needed a new carb & exhaust manifold. Mercury 9.9 & 15 was a reed, carb, exhaust swap, 20 & 25 looks to be only the carb. Yamaha 9.9 to 15 is only reeds (I think). These are all two stroke motors though.

It is an economy of scale; same block, same pistons, cowlings, lower unit, etc. The only difference being a cheaper part(s) of the motor.


----------



## TomFL (Oct 2, 2007)

> > Are you guys saying you can make a Chevette a Corvette just by changing the letters?
> >
> > There has to be more to it than that.  I just can not believe it is that easy.
> >
> ...


Even the new 4-strokes are like that. The merc 25 and 30 4-strokes are mechanically the same. Only difference is the computer programming. 

Merc wants $1200 for the program to bring a 25 up to a 30. 

Crazy

-T


----------



## Gramps (Nov 1, 2007)

Jim - I have a Bob's Mini Manual Jack Plate if you are interested...


----------



## jimbarn1961 (Aug 2, 2009)

Gramps u have a pm


----------



## jimbarn1961 (Aug 2, 2009)

Gramps, u have an email, lets talk and get er done


----------



## Flyline (Mar 3, 2008)

> > > Are you guys saying you can make a Chevette a Corvette just by changing the letters?
> > >
> > > There has to be more to it than that.  I just can not believe it is that easy.
> > >
> ...


not meant to hijack the thread........

do u know who or where I can take the hatsu 25 4-stroke to get it programmed to 30hp? any info? thanks.


----------



## Gramps (Nov 1, 2007)

Blake, the dealer? Or they know who would.


----------



## Brett (Jul 16, 2008)

[smiley=threadjacked.gif]

I wonder what the price is for a 30 hp 'hatsu brainbox?
No reprogram, plug-n-play... :-?


----------



## jimbarn1961 (Aug 2, 2009)

Thnx Scott, GOT ER DONE! 
Get with you next week as soon as you are ready


----------



## iMacattack (Dec 11, 2006)

Two spokes you can play parts monkey but IMHO not worth it.

On four strokes not as easy. I have talked to Merc-Hats-San (Mercury Tohatsu Nissan all use the same motor from 30hp down) and yes it's a brain box and a couple other small parts. Evinrude it's a brain box only. No one is reprogramming (burning) the chips. Unlike in the aftermarket automotive industry there is not market for to reprogram small motors. 

Oh and kiss your warranty good buy.


----------



## jimbarn1961 (Aug 2, 2009)

Thanks IMac,

My local dealer says easy fix just a carb swap and some adjustments. unfortunatly carb is $300 for a 25% or 5hp increase in power plus shop labor if u can't do it yourself. I've already decided to buy gramps jackplate next week and have a tiny tach on order already also. I am going to start by raising motor from current cav palte 1.5 below boat to plus 1" above bottom then trim boat read max rpms and see where we are at. Thanks for all the help guys. 
Now lets put this thread to rest


----------



## noeettica (Sep 23, 2007)

As an old VW tuner I would lower the Hp of a motor with different Venturis for day to Day and Then put in the Large tubes for Drag Racing ... 

Ran smoother with the smaller tubes ...

Dave


----------



## mark_gardner (Mar 18, 2009)

> Now lets put this thread to rest



put this thread to rest ??  hardley, this is just the beggining of a long drawn out saga of you trying to tweak a few extra ponies out of a 2 stroke  we're gonna need photos of the actual process as well as a review after the mod has been completed, we love stuff like this ;D


----------



## noeettica (Sep 23, 2007)

Nitro + Castor


----------



## jimbarn1961 (Aug 2, 2009)

OK I'll bite,  lets fire it up then and see where this goes. Do u think I could kill the thread by piercing it thru the piston with a (Wooden Spark Plug) ;D ;D 

*NEXT *


----------



## Brett (Jul 16, 2008)

Throw enough time and money at her
she could end up looking like this










That is one sweet hunk of outboard porn!
I just like looking at the way she sits there
not moving and stlil looking like she's gittin' it!


----------



## mark_gardner (Mar 18, 2009)

> Throw enough time and money at her
> she could end up looking like this
> 
> 
> ...


 let me know if you ever get your hands on one of those and we'll slap it on the back of the mud minnow, if shes still here  if not, the f & f said shes up for the challange   ;D


----------



## TomFL (Oct 2, 2007)

I can't believe nobody's brought up the "just slap the 25hp stickers on it and call it a day" routine. 

Don't stickers make the boat faster?



-T


----------



## mark_gardner (Mar 18, 2009)

mines so fast the peeled off  ;D


----------



## HighSide25 (May 15, 2007)

check out ebay. i got a good working 25 hp used carb for under $50 a few months ago. easy to install, just dont tear any gaskets or torque the two main bolts too much and youll be fine


----------



## jimbarn1961 (Aug 2, 2009)

Thanks Kyle! I started a daily search on ebay I am in no hurry so I will just wait until the right carb and price show up on ebay 

By the way maybe well see u IRl over thanksgiving week. The boys are going into fishing withdrawls real bad and need a fix ;D


----------

