912ULS Rough Sync At Low Revs

Hi all,

I've found a few other posts on roughness at idle and other rev ranges but I was wondering about my specific example. I have a Kitfox 4 with 912ULS (reversed manifolds and carbs) and I've recently rebuilt the carburetors with the rebuild kit from CPS.

After starting and warming up on the ground it will operate at 2200 - 3000 pretty smoothly. The plane performs well at 5200RPM, through the 3000-5000 rev range it's good, and when I pull to idle it's also pretty smooth.

The issue I'm having is that when I'm on downwind and pull to idle it's fine, but if I put in a little bit of throttle to get to 2200 or 2400 it runs really rough, like a dug-dug-dug-dug out of sync vibration. After I've landed and I'm taxiing it also runs rough between 2000 and 2500 RPM.

I synced the throttle cables pneumatically for 3500 RPM with a carbmate and set the idle screws at 1.5 turns out.

One thing I noticed during the carb re-sync is that the throttle butterfly valves close slightly differently i.e. when both throttles are closed, one butterfly is fractionally open while the other sits flush with the throat of the carb. Could this cause low-RPM roughness?

I'm wondering if there is something I'm missing or is this one of those 912 ULS things where because I synced at 3500 RPM the lower rev ranges will be rougher.

Thanks, any help greatly appreciated.