I've gotten around 30 witth a Weber on an L20B.
Only time I ever did it. Dunno what changed, but I've been doing about 20-25. $12 lasts me a day or two.
My 210 got nearly 40 mpg on the A15 with the Weber, MSD ign (made a nice difference) and ported head.
For your issue, have you checked the timing? I can get a smooth idle around 550rpm with a weber. and electronic ignition. I run about 14 degrees at idle with 89 octane to prevent pinging at high RPM's.
Clean idle jets? Primary and secondary idle jets are on the side of the barrels. Very easy to access. New fuel filter? Plugs, rotor, cap, wires?