the power piston stop or APT screw is set at the factory to give the engine the correct cruise a/f mixture. a good place to start is 3 1/2 turns from the bottom, then use it to adjust the cruise mixture. the way we set the cruise mixture is with a exhaust gas analyzer, the CO reading taken in front of any catalitic convertor and with any smog pump disabled is to target a CO reading of 0.75 -1.25% CO. (any smog shop should be able to give you the readings for just a few$),you could also use a air/fuel meter such as the Innovate Motorsports LM-1 and then target a air/fuel mixture reading in the 14.2-14.5 to 1 range. i know of no easy way to do the APT screw other than those methods or trial and error spark plug readings.
henry @ oles carb