He's likely running into a not so swift carb design. There's a spring that holds the carb barrel against a stop/follower that must be compressed as the carb closes. The groove that stop/follower travels in is not machined real well, making sticky installations pretty easily done (when closing/compressing that spring).

I'd start by changing over to a solid (2-56?) push rod of the type generally shipped with the kits. You might also have a look in the engine owners manual, at the initial carb setup, paying attention to the fact the engine idles with the carb open a little further than most designs. It doesn't need to close completely - not on the idle anyway.

Take your time here. It does need to work reasonably smooth to function well. Keep in mind that when the engine is running, the vibration may work to your benefit with this issue.... -Al