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Thread: yamaha 9.9 upped to 15hp ?

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    Default yamaha 9.9 upped to 15hp ?


    I installed a new reed assembly, 6 reed vs 4 reeds. I went over the parts for a 9.9 and a 15 and the reed assembly is the only different part.
    The 9.9 would run the boat 18 mph; after the upgrade the same rig will run 20.1 mph. This is with a 9.25 X 10 alum prop.
    My buddies boat with same upgrade runs 22.7 mph with him and me in boat!!!
    That is 500 lbs of fisherman. His boat is jon wood smooth bottom and heavy. Mine is 1442 alumacraft. bottom has ribs and rivits.
    I did tweak the mixture on the carb a bit, made it richer ~1/2turn as suggested in manual.
    Did I miss something? My buddy did his 20 years ago and thinks he did not change carb. Have any of you done this upgrade? suggestions?

    PS sounds little, but boy 3mph in a jon boat is a lot!

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    By adding the new reed cage you have added air. I would think that by turning the screw you just changed your idle mixture. Not being familiar with that carb I'm not for sure...but I would think to richen the mixture you would need to step up to a large jet. I think by doing so you'll pick up some. I really do not think the screw changed the mixture of the mains.

    Look at your manual or pull the carb and look at the numbers on the jets. You can probably play with a few jets until you get it just right. I would say when you are running you are probably running a bit lean. Do you premix?? If so that can be bad. I know the crossflow johnnyrudes can run without the breather attached and sound pretty sweet...maybe pick up just a hair on holeshot...but not noticable...the loopers though can burn up if you are running them tight without the breather. They lean out. After putting the reed cage in it you have added more air but without a jet change I don't think you have gotten all your potential and may be in a dangerous area without more fuel. I may be wrong on it...but I would check and see. Anytime you add more air...you need to add more fuel to get full potential...if you premix and have too much air...that spells for disaster. All I'm saying is make sure you have enough fuel for your additional air and flow.

    Check it out and let us know...then you can to play with the exhaust some...:D...maybe a hole in the midsection!! Would sound like a chainsaw on roids!!
    "But if serving the Lord seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your ancestors served beyond the Euphrates, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the Lord.” Joshua 25:15

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    oh yeah....get yourself a stainless prop...that will help!!

    if your boat is painted take some 400grit sand paper and scratch the bottom of the boat against the plane....widthwise across the bottom of the boat. This works on fiberglass hulls for a couple. Do this starting at the back and up about 4ft towards the front. What you are doing is roughing up the bottom...this causes air bubbles to form from the turbulance...hence you are more like a skipping rock...riding on air...lol...:D

    The old tale that waxing the bottom of the boat is not true. A slightly rough bottom is faster than a slick bottom...just like a slight chop is faster than a slick as glass surface.
    "But if serving the Lord seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your ancestors served beyond the Euphrates, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the Lord.” Joshua 25:15

  4. #4
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    Default jets

    Thanks for the feed back! the 1989 9.9 sf list main jet as #104 and Jet Pilot as #52.

    The 1989 sf15 list the exact same jets and part numbers. They have the same carb!

    The only thing in the manual different for the 9.9 and 15 is adjusting the idle mixture the 15 is 1/8 turn leaner.

    I tried a SS prop on a suzuki 9.9 4 stroke. I never got over 16 mph with that moter.

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