Which screw? On a multi jet carb like the pz you will have an idle speed adjustment and an idle mixture adjustment. Neither of these affect the overall fuel mixture. That is handled by the two jets and the needle. Up to 1/4 throttle the slow or pilot jet is what mainly affects fueling. From 1/4 to 3/4 the needle adjusts fueling from the main jet and emulsion tube. 3/4 to wide open is entirely the main jet.
Changing the main jet will slightly alter the needle position as you are changing the total fueling. This is why I tell people to leave the needle in the middle and save that adjustment for last.
The idle mixture setting is affected by the pilot jet in the same way. The pilot jet is by far the easiest to determine. Hold a steady 1/8th throttle and listen to the motor. 4 stroking a bunch, go smaller. Hesitant and jumpy/boggy, go bigger. From there you can set the idle mixture.
Also, always make sure to judge fueling on an engine that is fully up to temp. Doing anything else is a waste of time.
