It's an Encarwi Carb, I do believe...and yeah, it might be sucking air.
The slide in that only goes one way, and it's real hard to mess that up...
There's an idle screw on the left hand side of it, which might be your problem....try adjusting that.
In addition to checking the idle, timing and for possible air leakage - check your throttle cable settings...Jawa/Trac generally need a LOT of throttle pull to start, but not so much after that...so put some slack in the throttle and try starting it with the throttle held down a bit.
-Ree