Problem with idle speed screw
 

I have a 1.75cc Team Infinity engine on my buggy. As far as I know, the carburetor opening becomes larger when you turn the idle speed screw clockwise and becomes smaller when you turn it anti-clockwise. But my carb opening remains unchanged either way.
It works fine when I use the throttle and brakes on the remote.

Look down into the carb and then hit the brakes. Does it close when you do? If so, there is your problem. Fix the trim.

Yes it closes when I hit the brakes. But the problem is that the gap is not varying when I turn the idle speed screw.
I increased the trim and the gap increased marginally but no effect when I turn the idle screw. :(

the carb might need the servo to push the carb slider closed. I have a few engines that sit at 1/2 throttle without a servo attached from the spring of the rubber boot.(out of the buggy)
Try adjusting your linkages.

On your Carb linkage, there will be a spring pushing against the carb, Ensuring the carb is closing when the throttle linkage is closed, As wingracer said, you need to adjust your Sub Trim or your regular trim(pending radio) and ensure the carb is being closed with a little pressure behind it.