Carb sync issue

Hi!

 

We have trouble with the synchronization of the carburetors on our Rotax 912 ULS.

 

Both carburetors are synced mechanically first with no issue. Upon startup the idle speed was a bit high, so it was reduced. It now lies around 2000 rpm with is good for our Zlin Savage Cub.

Upon syncing with the pneumatic carbmate unit, we see some troubles.

At first it operates good at 2500rpm. Only minor adjustments is needed. We then have green light and no vibrations up to 5000rpm. When we slowly retard the throttle, we see that the carburetors split. When we pass 4000 rpm on our way down we get vibrations and a red light on the carbmate showing an unbalance almost off the scale. At low rpm and idle, it stabilizes again.

 

Checked the right hand carb, and found the rubber mount to be loose. The carburetor could even rotate in the mount. Tightened the clamp, it got better, but still we cannot sync with good results. Already ordered new rubber mount and o rings in case there is a vacuum leak (I suspect it).

 

I must also say that we cannot see any binding in the cables, linkages or anything… everything runs smooth.

 

Is there any known «must check/must do» when you see these kind of troubles when performing the pneumatic balancing?

 

We have all the manuals, and even access to the Rotax owner videos online.