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Carb Synch doesn't hold thru RPM range

in 912 / 914 Technical Questions 12 years ago
912ULS. 400 hours. Doing carb synch with Carb Mate, have done several times before. I can get the carbs synched perfectly at 2500 RPM but as I increase the RPM's to over 3500 the synch does not hold. Can anyone help?

  • Re: Carb Synch doesn't hold thru RPM range

    by » 12 years ago

    Its likely your cables: en-equal length? tiewraps binding one? Tight bend on one cable? Broken strand inside? Sticky? Swivel binding?
    On our Europa it was not repairable; we need to replace the original cables with new, friction-free cables.
    Thank you said by: Robert Hirschfield
      
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  • Re: Carb Synch doesn't hold thru RPM range

    by » 12 years ago

    Hi Robert

    That is what your balance tube is for.

    You can only set the idle screw so the vacuum is equal at tick over, and any RPM on the cable adjusters. As you increase or decrease the revs, they will become out of balance.

    Mark
    Thank you said by: Robert Hirschfield
      
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  • Re: Carb Synch doesn't hold thru RPM range

    by » 12 years ago

    Hi Robert,

    It can be as Rob suggest, but this is exactly why I tell people to quit using the too low 2500 rpm to set up for the higher rpm sync. It is too close to the idle circuit rpm and doesnt flow enough fule and air to get things setup accurately for the higher rpm sync. Use 3500 + rpm. Then you won't have all the vacuum change as you throttle up. Read this:

    http://www.rotax-owner.com/rotax-blog/item/16-the-anatomy-of-a-carb-sync

    This should help.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Robert Hirschfield
      
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  • Re: Carb Synch doesn't hold thru RPM range

    by » 12 years ago

    Thank you for the replies. I am doing the procedure correctly, it has worked flawlessly in the past. I am going to take a very close look at my cables and go from there. Thanks again.
      
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  • Re: Carb Synch doesn't hold thru RPM range

    by » 12 years ago

    Robert

    You should set your idle screws so that the vacuum is the same at around 1600-1800 rpm, then set the cable adjusters so the vacuum is the same at around 2500 rpm.

    The vacuum should be "near enough" throughout the rev range, and any slight difference is compensated for by the balance tube.

    Setting the cable adjusters at a higher rpm of around 3500 can mask underlying problems as to why the vacuum is not stable through the rev range and is not recommended by Rotax.

    Mark
    Thank you said by: Robert Hirschfield
      
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