K10 mixture ratio

Took my k10 to a mechanic to set the timing. Found that the vacuum on the dizzy isn't working so have ordered a replacement. He also checked the O2 and claimed that it is running rich. He wanted to adjust the mixture screw on the side of the carb, but didn't have the correct tool. I know these are pretty well setup from factory, would there be a better way to adjust the ratios to back off the fuel slightly. Thanks in advance