I have done carb synchronization on two RV-12’s with 912ULS using CarbMate electronic synchronizer with good success.  Synchronization in each case was completed within about ten minutes and engines run smooth and idle 1400-1500 RPM.


Fast forward to today when I’m helping a friend synch his 912ULS RV-12.  When connected, the CarbMate shows huge imbalance with indicator all to one side.  If I manually move each throttle arm by hand a little bit to determine which carb is producing power, and which one isn’t, nothing changes on the CarbMate scale – its pegged to one end.  This is weird.   I reverse hoses to the CarbMate and the scale switches ends so CarbMate is functioning correctly.


Looking at the idle adjusting screws it appears that the left carb is completely closed by the throttle plate itself and the idle speed adjusting screw is lifted off the idle stop.  I have adjusted cables to make sure throttle pulls the arms against the idle stops and yet it appears the throttle plate is what is actually setting the idle position and not the idle stop screw.


So, in addition to huge vacuum imbalance I can’t get a low idle.  Engine is running 1800 – 2000 RPM.  Right carb can be adjusted to lower idle speed but left carb cannot.  I will check cable lengths and mechanical setup per Van’s instructions.


Engine runs smoothly at higher speeds and CarbMate shows good balance at 3000 – 4000 RPM.


Any idea what could be causing such huge vacuum imbalance at idle?

  • Re: 912 Carb Synch Issue...

    by » 8 months ago

    Did you perform a mechanical synch before the pneumatic synch? This is adapted from the video on this website with references to the diagram in the service manual:

    Mechanical Synchronization

    1. Remove return spring (5) from throttle arm
    2. Remove throttle cable (4) from throttle arm (1)
    3. Move throttle arm to idle position
    4. Clip return spring to hold throttle arm in idle position
    5. Unscrew idle stop (2) until a gap can be seen
    6. Set gap to 0.1 mm (0.004 in)
    7. Turn idle stop screw 1½ turns clockwise
    8. Place return spring back in its original position
    9. Gently close idle mixture(6) screw by turning clockwise until it stops
    10. Reopen by turning 1½ turns counter clockwise
    11. Repeat on second carburetor
    12. Re-connect throttle cables (4) and check for full movement from stop to stop
    13. Check that cables hit stops simultaneously
    14. If they do not, adjust at throttle arm
    15. Start engine and verify idle speed
    16. Set idle stops equally so that engine idles at 1800-2200 rpm (min. 1400 rpm)


  • Re: 912 Carb Synch Issue...

    by » 8 months ago

    Excellent.  Thank you...

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