Adjusting the idle screw does not actually adjust the idle speed, it adjusts how closed the carb can go. If you adjust it, yes the carb will be able to close more but unless you then adjust the throttle linkages so that at idle it is closed more you will not see any effect. If you have the idle screw so loose that when you put the brake on it is closing the carb completely it will cut the engine out.

What you need to do is turn on the controller but not the engine.

Take off the throttle servo horn and put it back on so that the servo horn is pointing straight towards the engine so it is at 90 degrees to the servo.

Take the air filter out and look down into the carb.

Undo the little stoppers on the throttle rod going from the servo to the engine.

Slide the rod through the servo horn connector so that the carb is just 1mm open.

If you are not able to close the carb enough then that is when you would turn the idle screw a little anti-clockwise which will allow you to close it more.

If you do not have to adjust the idle screw and can close the carb so it is just 1mm open after you do this then turn the idle screw clockwise until it starts to try and push the carb open.

Once you have done this tighten the stoppers up so that they hold everything in that position and then you may need to adjust the stopper next to the brake so that the brake only comes on when you push forward on the controller.