that top part is the butterfly valve aka the throttle plate which when turned
counter clockwise increases the speed of the engine that screw that you messed with
when turned clockwise will increase the speed of the engine(idle speed )
that bottom screw that you didn't mention is the mixture adjustment screw.
If you haven't messed with it ......don't......If you have ....we need to talk some more :smile:
cheerfully,
Normf