912 Carb Synch Issue...

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?