How do you adjust carb on a Craftsman 20cid chainsaw?

There are H and L adjustment screws on the carb. They might be splined so you will have to notch them with a hacksaw blade or a dremel so you can use a flat head screw driver.

The high speed jet is used to richen or lean the fuel to air ratio at high speed and the low speed jet is used to adjust the fuel to air ratio during idling. Your details indicate that you need to adjust the high speed jet for more fuel. Start by turning the high speed jet 1/8 to 1/4 of a turn counterclockwise (CCW) and then start and test operation. High speed jet "H": The high speed jet screw "H" influences the chainsaws power and engine speed. A correctly adjusted carburetor means that the saw accelerates without hesitation and the saw burbles a little at full throttle. In addition the chain should not turn when idling. If the "L" screw is set too weak this can make the saw difficult to start and cause poor acceleration. If the H-screw is set too weak the saw will have less power, poor acceleration and/or motor damage. An over rich setting of either L or H screws will cause acceleration problems or too low working speed.