yahama 2 stroke from 4 to 5 hp

Discussion in 'Outboards' started by patru alexandru, Oct 23, 2019.

  1. patru alexandru
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    patru alexandru New Member

    hello my name is alex and i am the owner of an 4 hp 2 stroke yamaha (1991-2000)and was wondering if a simple carburetor change can boost it to 5 hp?
    alone with no gear i can plane with my boat an reach something like 22 km/h but with fishing gear in that its imposible... that extra hp is the thing that does the trick. i have tested a 5 hp (same generation 2 stroke) and iti does the job well.
    can it be done? is it just a jet change or full carburetor ? has enyone done this tune-up ?
    thank you
     
  2. DCockey
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    DCockey Senior Member

  3. ondarvr
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    ondarvr Senior Member

    It is NEVER just a jet change.
     
  4. DCockey
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    DCockey Senior Member

    But it can be a simple as a different carburetor, or a restriction in the intake system.
     
  5. Ike
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    Ike Senior Member

    Frequently with outboards the differences may be as simple as a carburetor. It's basically the same engine, just tweaked to get more HP. My 4 HP mercury comes in 4 HP, 5 Hp and 6 HP , and it's really a Tohatsu. They all weigh the same, have the same cubic inch displacement, gear ratios, and prop. So as David said check the specs, and see what they say. By now the warranty has run out so any changes you make won't void the warranty.
     
  6. patru alexandru
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    patru alexandru New Member

    Checked the specs o_O the jets seem to be the only different pieces that could affect the power (screw's dont count). i dont know what else to look for :(:(:(
     
  7. ondarvr
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    ondarvr Senior Member

    The jets are chosen after everything else is set in place to achieve the designed hp.

    The jets keep the fuel to air ratio in the correct range, so assuming the correct jet is in the carb now, a bigger jet does not increase the HP, it will typically make it run worse.

    If other changes are made to the motor, then a jetting change may be required to keep the fuel to air ratio correct.

    People tend to notice a jet may be different, then assume it is the jet change that increased the hp, but in reality the different jet is required because some other component(s) was changed that required the different jet.
     
  8. DCockey
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    DCockey Senior Member

    ondarvr is correct. Different jets imply something else is different which affects airflow, probably the carburetor body. The parts book presumably does not list the carburetor body separately so a difference in carbuertor body would not be apparent from comparing parts book lists of carburetor parts.
     
  9. patru alexandru
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    patru alexandru New Member

  10. Zilver
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    Zilver Junior Member

    In the Cylinder and crankcase list there's also a difference : bolt nr. 28 and 29. Maybe one of them is longer and partly restricting the exhaust ? Hans
     

  11. DCockey
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    DCockey Senior Member

    The crankcase and crankshaft/rod/piston are also different. Probably the displacement is different.

    As ondarvr said a difference in the nozzle and jet alone will not increase power. There also needs to be a difference in the airflow which the differences in crankcase and crankshaft/rod/piston would provide.
     
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