Outboard carburetor nozzle/jet consumption?

Discussion in 'Outboards' started by ryanonthebeach, Jul 3, 2009.

  1. ryanonthebeach
    Joined: Mar 2007
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    ryanonthebeach Junior Member

    The Yamaha 9.9 4 stroke outboard engine looks identical to the 15 except for a few bits and bobs on the carburetor, Diaphragm, nozzle and jet, same bore stroke etc.. If I upgraded it to a 15 would it use more fuel at the same RPM, all other things being equal?
    I’m assuming an outboard’s governor varies fuel flow to keep a constant RPM. Are there any other factors related to the jet that would decrease use more fuel?

    Because of the size of the boat and the speed it will operate at, the prop that came with the outboard is perfect for 15 rather than 9.9. So it will hold the RPM’s down a bit too much at 9.9, either I’ve got to buy a new prop with less pitch or upgrade the valve to 15.

    By my calculations (with the stock propeller) at 3700 RPM I would be using 5HP with around 7.7 HP available from the engine. With the 15 jet would it use the same gas at this speed/HP as with the 9.9 jet?

    At the upper end, the stock prop has a bit too much pitch, so will peak early and will hold the RPM’s down at around 4700 in fair conditions which is below the middle of the operating range of 4500-5500, but with 15HP available it’ll be peak at 5450 RPM’s which is close to max of 5500 and will allow a decent margin for rough conditions.
    Here are the details of the engine installation prop etc.
    Propeller selection boat repower
     
  2. thudpucker
    Joined: Jul 2007
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    thudpucker Senior Member

    This thing about changing Carbs and magically you get more HP keeps coming up.
    Mostly its a rumor.
    You can find out for sure by looking at part numbers.
    The rule for Gas engines is 14.2:1, or A lot more air than fuel.
    Just changing a fuel supply device is not going to change that. If you have a 15 it will use a lot more fuel than a 9.
     
  3. Frosty

    Frosty Previous Member

    No,-- some engines are de tuned to reduce power such as 100% sure of the 2.5 and the 3.5. I always buy the 2,5 and open the carb slide restricter. The last Merc 2.5 I bought had gone a little more technical by making the slide longer so it could never open fully. This was a bit more technical to detune but is possible , just saw the slide down.

    This is restricting air/ fuel supply not 14:1 mixture

    Outboards do not have governors.
     
  4. thudpucker
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    thudpucker Senior Member

    I was going to say that he might have a 15 with a 9.9 Carb and window dressing. I didnt have the eloquence to get that techincal with him.

    I do believe in EFI and its ability to deliver the Same HP as a Carb with much better fuel economy because it knows when the engine dont need so much fuel.
     
  5. Frosty

    Frosty Previous Member

    I don't want EFI on a engine that will be out in the rain or dumped in the sea occasionally.

    Theres a place for EFI, like on your car,-- it aint on my outboard.

    I like the 2.5/3.5 I can put crank bearings in In 1 hour,--done it.

    I often have to clean the carb with their tiny jets . Water is like treacle to them. No sir I wont have EFI.
     
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    <many motors just have a stop in the throttle to keep them from producing the HP. Others have different carburators. On outboards with mechanical spark advance it can get a bit tricky to set it up properly. You have to modify the slide too.
     
  7. ryanonthebeach
    Joined: Mar 2007
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    ryanonthebeach Junior Member

    Outboard carburetor nozzle/jet consumption

    Thought it was a simple question, guess I was wrong, let me try again.

    Assuming this modification works (air and fuel) at a given RPM’s (say 3000, 5hp used) with all else being equal will the 15 use more fuel than the 9.9?

    I’ve gone though all the engines parts and all parts save for a handful on the carb are EXACTLY the same for the 9.9 and 15.
    Except for a diaphragm assembly which is not on the 9.9 but on the 15 but fits on the carb. Main nozzle, main jet and associated caps and seals (see pics).

    The parts that are different on the 15 are:
    5-13, 22, 23, 25 34

    9.9 Carb
    [​IMG]
    15 Carb
    [​IMG]
     
    Last edited: Jul 4, 2009
  8. Frosty

    Frosty Previous Member

    It shouldnt use more fuel. the rule of thumb is 5 GPH per 100HP used
    for diesel 7 Gph for petrol and 10GPH for 2 strokes.

    So if you doing the same speed even with a bigger engine mathematically it should not use more fuel.

    Some cars with bigger engines use less than the smaller version.

    Has anyone had a 9.9 on a dyno? or a 15. Who knows what they are.
     
  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    0n a dyno they are, by law, what they claiim
     
  10. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The only difference seems to be the size of the jets.
     
  11. ryanonthebeach
    Joined: Mar 2007
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    ryanonthebeach Junior Member

    Thanks!
    Do you know what that diaphragm thingy that's hanging off the bottom on the 15 carb is for?
     
  12. Frosty

    Frosty Previous Member

    I don't think anyone would be too upset if their 9.9 turned out 12HP.

    Using bigger jets cant do anything but richen it, less power and more fuel consumption. cant be just that.
    The carb body is different and could have a larger venturi.
     
  13. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    All engine manufactures use a "base" engine and up grade it for a few different models. How they upgrade is going to vary with each package and manufacture. Some use stops, others jets, others timing, etc.

    The easiest way to find out what you have is to look at the engine specs and part numbers.

    An example might have a 9.9 with the same stroke and bore of a 15 HP. This is a good indication the 9.9 is the base for the 12 and 15 HP models of this product line. Check the carb rebuild kits. Typically they'll have two or three different high speed jets in the kit and a quick check of the numbers on the jet suggest which is the "big" one, though you can usually see the difference in orifice diameters. Also check the tune up specs, which may show different timing for the larger models.

    The difference between the 20 HP Johnson I have and the 25 and 30 HP models of the same base engine, is two jet sizes and 3 degrees of timing. My 20 is now a 30 HP with a new jet and some timing advance. This is commonly done.
     
  14. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I think that diafragm thing is an enricher or accelerator pump. What does the parts list call it?
     

  15. ryanonthebeach
    Joined: Mar 2007
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    ryanonthebeach Junior Member

    Gonzo
    The part names are as follows:
    10 - DIAPHRAGM ASSY 1
    9 -COVER, DIAPHRAGM
    This is attached to a
    34 - CHECK VALVE ASSY

    Thanks PAR, I'll take a look into the timing to make sure.
     
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