I have tried to expand the image of your carburetor, in the hope that someone may be able to nominate the manufacturer and model. I don't recognize it - most likely it is some model of Walbro, Tillotson, or Zama. If you have the operator's manual for the equipment it will probably tell you how to adjust the mixture. If it doesn't, it will be in the carburetor manual, which is probably available on this site.
If you can't get this information, you can make a generic tuning adjustment as follows. One of the two screws in the picture will be low speed, the other high speed. The usual approach is to begin by carefully noting the current setting on each screw (that is, you note the angle of the screw slot, then turn the screw clockwise gently until it bottoms, noting how far it rotates. Then you put it back exactly to its current position.) Second step is to run the motor with the throttle set in the start position, and adjust one of the screws (I'd start with the one on the right, but I could be wrong) and find when it runs fastest and most smoothly. Find the leanest (clockwise rotation) and richest (anticlockwise rotation) settings before it starts to run worse, then set it halfway between them. If this adjustment makes hardly any difference, return it carefully to its original position then switch to the other screw - it must have been the slow-running mixture, not the high speed mixture. When you have got it running as well as possible at high speed, release the throttle from the start position, and set the other screw to give the best idle you can get. If you had to adjust the idle screw very much, recheck the fast-running screw afterward.