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

Discussion in 'Outboard Maintenance' started by jimbarn1961, Nov 12, 2009.

  1. jimbarn1961

    jimbarn1961 GOT FEESH?

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    Just bought new skiff, maiden voyage runs great. Just need more speed SCOTTY :D. 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.
     
  2. Brett

    Brett > PRO STAFF <

    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
     

  3. Brett

    Brett > PRO STAFF <

    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.
     
  4. jimbarn1961

    jimbarn1961 GOT FEESH?

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    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.
     
  5. Gramps

    Gramps Living &amp; Dying in 3/4 Time

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    Don't dump money into it yet Jim!! :D 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
     
  6. jimbarn1961

    jimbarn1961 GOT FEESH?

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    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?
     
  7. rkmurphy

    rkmurphy Well-Known Member

    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.
     
  8. jimbarn1961

    jimbarn1961 GOT FEESH?

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    RK,

    Does that knowledge make you the previous owner? ;)
     
  9. rkmurphy

    rkmurphy Well-Known Member

    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! :)
     
  10. Brett

    Brett > PRO STAFF <

    marine salvage shop might save you a few bucks

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

    TomFL Well-Known Member

    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
     
  12. jimbarn1961

    jimbarn1961 GOT FEESH?

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    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
     
  13. DuckNut

    DuckNut Brandon, FL

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    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.
     
  14. Brett

    Brett > PRO STAFF <

    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.
     
  15. mark_gardner

    mark_gardner I Love microskiff.com!

    i have a buddy in boynton bch with a 25 merc 2 smoke looooong  shaft  :D ;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  :cool:
     
  16. Gramps

    Gramps Living &amp; Dying in 3/4 Time

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    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.
     
  17. TomFL

    TomFL Well-Known Member

    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
     
  18. Gramps

    Gramps Living &amp; Dying in 3/4 Time

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    Jim - I have a Bob's Mini Manual Jack Plate if you are interested...
     
  19. jimbarn1961

    jimbarn1961 GOT FEESH?

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