Hi Paul.
It might be a bit difficult to detect a big differance when adjusting the air/fuel screw as it makes a small differance to the overall running of the engine. An air leak around the inlet rubbers will also affect air screw settings...this can be checked by squirting damp start/easi start/WD40 around the inlet rubbers with the engine running. If the engine sounds as though it's increasing in reve slightly, that's where the air leak will be.
From what I remember you should start with the screws turned out by 1.5 turns from lightly seated. The manual then states to turn the screw out until that cylinder faulters, but i've only found this procedure of any use when doing a twin or a single, as on a four the other cylinders seem to compensate to a degree.
If you start with 1.5 turns out, then check all other settings, then balance the carbs, this should give you a smooth running engine. Then, if you feel that the engine is not that responsive from tickover, i'd remove the carbs & check the float heights. It's the air screw & float heights which have the most effect on an engine at tickover, then at about 1/8th throttle the primary jet kicks in...but the air screw continues to affect fuelling all of the way through to full throttle, as does the primary jet.
