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  1. #1
    Charlie M.'s Avatar
    Charlie M. Guest

    Default As I am slightly short of pow

    As I am slightly short of power I would like to upgrade a 25 to 30 or 35. Weight and transom strength will not allow a 40. Any opinions?

  2. #2
    Join Date
    Nov 2005
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    424

    Default " What's the boat rated fo

    " What's the boat rated for? Are you planing on buying new, or finding a particular used engine? An increase of 5 HP really won't mean much in performance, you're going to have to got to at least a 35 for any noticeable difference. If you have weight concerns, it's going to have to be a 2-stroke as well.

    A few ideas:
    Nissan 40 HP 2-stroke 3 cyl - 160 lbs
    Mercury 40 HP 2-stroke 2 cyl - 175 lbs
    Johnson 50 HP 2-stroke 2 cyl - 200lbs

    The Johnson 50 HP is also available for the same weight in a 40 HP. There really are not any new 35 HP's out there, you could find one (mabye new or used), but I'd just worry about weight - if a 35 & 40 HP weigh the same, may as well get the extra 5 HP.

    Jon "

  3. #3
    CharlieM.'s Avatar
    CharlieM. Guest

    Default " The boat is rated at 25hp.

    " The boat is rated at 25hp. When you put on a Jet Pump, you loose about 30%, so 40 is over limit (28.8hp). 35 (24.5hp) would be great, but none are available new. That's why boosting a 25 (common motor) is in play. Thanks much! "

  4. #4
    Join Date
    Dec 2002
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    294

    Default "Jon;If you put John/Rude same

    "Jon;If you put John/Rude same year 1990 35/30hp carb and intake manifold on your 25hp you have added that much horsepower as that is the only difference between a 25hp and a 35hp with no increase in weight! [email protected]"

  5. #5
    Johnnielund's Avatar
    Johnnielund Guest

    Default " Charlie; I have a 1990 25 hp

    " Charlie; I have a 1990 25 hp Johnson that the previous owner had converted to a 30 hp by a very reputable marina here in northern Illinois. I have a 16?Lund Explorer and am very happy with this boat/motor combination. According to Joe Reeves the 25 and 30 have the same block, cyl head, crankshaft, rods, pistons, leaf plate, leaf valves etc. The differences between the two are as follows:

    The throat hole venturi on the on the carb and manifold is larger on the 30 than the 25.

    30hp is hit at 5500 rpm. 25hp is hit at 5000 rpm.

    The cam, which is attached to the magneto armature plate, is different. The 30hp cam is # 326876. The 25hp cam is # 334750.

    The orifice at the extreme top of the carburetor is different. The 30hp is # 39 (.039) # 334568. The 25hp is # 30 (.030) # 317473.

    The high speed jet, located at the center bottom portion of the float chamber is different. The 30hp is #65D (.065) #327541. The 25hp is #51 (.051) #331567.

    I hope this information is helpful to you. It is my understanding that trying to increase hp is usually not a good idea, however I think in the case of this particular motor it works well. "

  6. #6
    CharlieM.'s Avatar
    CharlieM. Guest

    Default " Johnnielund, Thanks much fo

    " Johnnielund,
    Thanks much for the information, and the time it took to compile it! Extraordinary!
    Sounds like it would be best to purchase a new 30 Carb, Cam and Intake Manifold to make the conversion.
    From anecdotal info from one of our local Johnson mechanics, the replacement part for the Carb is a p/n 439389, which comes with all of the parts you mentioned. Only p/n left to find is the manifold, and I'm running.
    Thanks again, "

  7. #7
    Johnnielund's Avatar
    Johnnielund Guest

    Default Charlie; I looked in my parts

    Charlie; I looked in my parts catalog for intake manifold part number. Part number is as follows: #396790. Good luck. Keep us posted on how you make out.

  8. #8
    Johnnielund's Avatar
    Johnnielund Guest

    Default " Charlie; When I checked on P

    " Charlie; When I checked on P/N for the intake manifold, I happened to notice the P/N for the 30hp carburetor is 432705. Check on that P/N. Hope my catalog is not wrong. "

  9. #9
    CharlieM.'s Avatar
    CharlieM. Guest

    Default " Johnnielund, Thanks much fo

    " Johnnielund,
    Thanks much for the manifold P/N! Hope to place the orders for all today, and have running by some time next week.
    I rechecked the p/n on the carb with our local John/Ev. dealer and the 432705 has been replaced by the 439389, which is listed for all horsepowers. Probably comes as a kit with all of the various jets and venturis.
    By the by, it appears that the reason this motor has better performance than most is due to the 32 CI displacement. A Merc. 20/25hp has 24 ci, the Nissan 25/30hp has 26. From talking to various outboard jet users, the additional displacement provides the torque to get the motor up to the rpms where their horsepower lies (5k-5.5k)? This is the same displacement as the old Nissan 40C, which by all accounts is a screamer, and only weighs 130#. "

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