If you feel your 50-cc carburettor needs to be adjusted, you'll be able to do this yourself with a screwdriver and a tachometer. The main things to consider changing are the idle speed and the fuel-to-air ratio, which can be affected by a number of things such as climate and altitude. The procedure is not difficult and consists mainly of trial and error.
Locate the spring-loaded screw by the throttle control. Turn the screw until the engine reaches an idle speed between 1500 and 2000rpm on the tachometer.
Locate the slotted idle mixture screw, which should be on the side of the carburettor near the air cleaner, to change the fuel-to-air ratio. Make sure the engine is switched off and turn the screw until it is fully in, counting the number of rotations. This number is your baseline so you can return the engine to its original state if required.
Return the screw back to the baseline and turn on the engine. Rotate the screw clockwise in quarter turns until the engine drops in speed. Make a note of how many turns this takes.
Move the screw counterclockwise in quarter turns until the engine picks up and then lowers again. Divide this number of turns by two to find the best point, and turn the screw that many times.
Rotate the screw in 1/4-turn increments to get it to its optimal stage.
Check that the idle speed screw has not gone above 2000rpm, and if it has, then adjust until it is back in the required zone.