Does turning the carburetor adjustment screw counter-clockwise enrich or lean the mixture?
It enriches it because acts as a flow valve on the pilot jet circuit. Basically, gas is pulled up through the pilot jet and and comes out some tiny holes at the base of the throttle plate aka butterfly valve. Then the passage continues where it runs into the "pilot screw" or "idle mixture screw". It then comes out one large or several small holes in front of the butterfly valve (this is the second place that gas enters the carb from the pilot screw). If that screw is turned in all the way (clockwise) flow is reduced to zero (at the second location only) the first l set of holes still gets gas.
TIPS:
- When ever you turn the screw do so 1/2 turn because it makes it easy to count. 1/2, 1, 1.5, 2, 2.5 etc.
- Always turn the screw clockwise and count the turns and record that number so you know the exact position before making adjustments. When you "seat" the screw don't use a lot of force or the tip of the screw will be damaged.
- The max fuel delivery is typically 3 turns counter-clockwise - turning more will NOT add more gas because the screw is completely out of the way and turning more than three turns can cause the screw to fall out while driving!
There is another type of pilot screw which works the same but in reverse. It is an "air bleed" screw. That reduces the amount of gas by allow air into the mixture. So it works opposite. Turn it all the way in for the max amount of gas. Turn it 3 turns out to dilute the pilot jet fuel by ADDING AIR to the mixture. Same basic effect but two different approaches to solve the same problem.
Final thoughts... as a scooter ages and sits there is a tendency for gas to gel and crystallize especially in the pilot jet and pilot jet passage which is tiny compared to the main jet. The result is LESS GAS (lean burn, hot and reduced power). So over time it is likely you will need to compensate by increasing the amount of gas from the pilot jet by opening up the screw but the reverse never happens. There is no way for the carb to deliver more gas unless the jets are changed or a passage is drilled out.
To see this in action watch my carb videos:
Carburetor Cleaning Part I
Carburetor Cleaning Part II
Carburetor Cleaning Part III
Kymco Carburetor Remove & Reinstall
