"c" is the idle adjustment screw. Closing it will cause the barrel to be open farther, when the throttle is cut all the way back. Thus causing it to be idling faster. If the screw was long enough, you could tighten it down enough that the barrel wouldn't move. Don't want that!!
If you have turned on that, try to get it where it was. You should be able to get a 2mm allen wrench in the carb throat with the transmitter at idle.