I believe the duckbill is there to restrict the air from entering your crankcase, but allowing pressure to vent. The only 4 stroke bikes I have ever owned that did not have some form of checkvalve were big twin Harley's. They had a rotary valve built into the oil pump drive. A whole lot of trouble, kinda think it may serve a pourpose.
If I remember correctly, the old 250 Ducati Singles and the Royal Enfield 250 Crusaker ran a open crankcase breather off there oil tank.
The breather duckbill isn't as critical on the AVL as it is on the old Iron Bullet.
Here's why.
With an unchecked breather hose, the air is pumped out of the hose when the piston descends, and pumps air back in the hose when the piston ascends.
This creates pumping losses which can slightly affect horsepower and heat generation.
Additionally, the Iron Bullet uses the low pressure zone in the crankcase to draw fumes down from the head, to assist the breather route.
A low pressure zone in the crankcase reduces pumping losses, helps rings seal better, and has less internal pressure to cause oil leaking out any gaskets or seals.
In the AVL or Iron Bullets with the oil tank breather, it is not as critical because that crankcase never really gets pumped-down to low pressure, so there never is a real low pressure zone in those bikes.
If it were an Iron Barrel model with the crankcase breather on the engine case, then I'd use the duckbilll because there would be an improvement.
With the AVL and later model Bullets with the oil tank breather, I don't think it will make much difference, so I removed it.
Cheers,
Bill