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I would like to know why Rotax decided to use two carbs instead of one.  I have read that it gives better power I also have read that the engine will run on one carb enough to keep you in the air.  I do not see how that is possible as each carb is feeding one half of the engine with no cross over allow fuel into the other pair of cylinders.  The use of two separate carb/cyl arrangement increases the chance of engine failure as I do not see how the engine will run on just two cyls on the same side of the engine.  I would loved to hear that I am not correct on this assumption please reply. I see the Jabaru engine with 125HP running on one Bing with larger jets and wonder why Rotax cannot do the same.   

  • Re: Two Carbs instead of one

    by » 4 years ago


    See attached figure and think how carburetors can installation canbe possible.?

    100 years efforts by Rotax can not go in to sink.

     

    25335_2_images 3.jpeg (You do not have access to download this file.)

  • Re: Two Carbs instead of one

    by » 4 years ago


    I would like to know this as well. I've heard the duel carbs being touted as redundancy for safety but if each only feeds two cylinders then what's the point?


  • Re: Two Carbs instead of one

    by » 4 years ago


    Once again, I am going to try to read the mind of the designers.

    Anyone who has ever sat behind a Radial engine knows it is the worst design that can be for visibility over the nose.

    The ideal profile is to have as little engine as possible above the prop shaft.

    The 912 series is pretty good in that respect due to the gearbox raising the prop shaft well above the crankshaft.

    The next consideration is how to get fuel into, and exhaust out of, the engine.

    Assuming you want to place a muffler below the engine, a bottom exhaust would be preferred, making a top intake the default location.

    To keep the profile low, Two small Carbs tucked in close would be preferred over one larger central carb.

    Compare this with a Continual style engine with the prop shaft being an extension of the crankshaft causing half the Block to be above the prop, limiting forward visibility.

    Keeping both the Intake and the exhaust below the engine requires an Up-Flow Carb and no room for any practical muffler system.

    Overall it just comes down to balancing out the compromises.  

    The quirky synchronizing of dual carbs is balanced by the improved forward visibility.

    Be happy they didn't go with 4 carbs, one for each cylinder!

    No Matter what they did, someone wouldn't like it!


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


  • Re: Two Carbs instead of one

    by » 4 years ago


    That makes sense. If one of them fails though, will the engine still run even if poorly or would it just stop?

     

    I'm guessing having each cylinder fed by two carbs in the same way they are fed by two spark plugs would not be practical?


  • Re: Two Carbs instead of one

    by » 4 years ago


    It's for the same reasons that old race cars and sports cars usually had multiple carburetors.  They basically delivered more, smoother power, which, in those applications, was considered worth the trouble of making sure they were all tuned/synchronized.


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