The idle screw will not change air flow through the idle tube by testing it the way you described. The concept is hard to put into words, but here it goes. The idle screw is a pressure valve, not a flow valve. The idle tube is like a straw which fuel is sucked up from the carb bowl, with the engine supplying suction, and the idle screw changes the amount of suction at the idle tube. However, the idle screw will not directly control flow through the idle tube because they are not connected to each other. The idle screw changes air pressure within the idle system. It does not function as a flow valve for the idle tube. That's why the air flow didn't change when you blew into the idle tube whether the idle screw was open or shut.
Your idle screw should not have any grooves around the beveled portion, grooves will not allow the 'fine tuning' necessary to adjust the idle mixture. I don't think the tip being pointed or blunt will affect anything. It wouldn't hurt to compare idle screws or see if switching them makes a difference in your Cub's idle performance. Closing the idle screw reduces the suction which reduces fuel flow to the engine, therefore the idle screw is turned in to lean the mixture (no suction at the idle tube = no fuel) and out to make it richer (lots of suction = lots of fuel).
I hope this helps and not just confuses it more.
Last edited by Lurker Carl on Sat Sep 04, 2004 12:07 pm, edited 1 time in total.