Hmm i have trouble with mine too and it always seems either too high or too low. If i wind the screw in too far it stalls though so could yours be jamming or is it running too rich?
After starting cold mine also revs up and down for about 20secs before settling down- do you reckon its a WUR problem? Ive seen a diagram of how they work (made no sense

) and not sure whether to get it modded.