Years ago people would use the throttle stop screw to limit the minimum speed setting of the engine. Obviously yours is screwed in too far because you should be able to close the throttle enough to idle it at 2500. Additionally you should be able to run the trim down on your transmitter to shut the engine off.

The little screw in the center of the throttle arm is the idle mixture screw. The big needle is the high speed mixture screw.

If the idle stop screw vibrates out, the carb barrel will fall out and the barrel spring will be ingested into the engine and ruin it. So don't screw this one ot farther than what is needed to get the engine to shut off.