Lean on idle in Carter W1, WA-1, WO, WDO 1-barrel carbs
Restricted metering hole in low speed jet. Replace if possible.
Air by-pass or bleed holes in casting oversize.
Restriction in passage in casting from low speed jet to port hole and idle screw.
Economizer hole in casting restricted.
Port opening too small.
Port hole restricted.
Hole restricted in casting where idle screw seats.
Threads in casting carbonized and not lettign idle adjusting screw regulate the amount of air needed to adjust carburetor (on high idle carburetors_
Air-leak at flange gasket
Idle screw burred
Port plug loose, or not seated in casting
If lever is loose on throttle shaft, it is hard to get idle adjusted correctly