A few possibilities come to mind
Easiest and most probable:
Unplug your vacuum advance. The wires on the pick-up coil to the ignition module are very thin, very brittle(with age) and move back and forth constantly. I've seen this before in the shop... Idles good but when the throttle is moved, vacuum starts to apply the vacuum advance circuit and the wires loose contact.
Leaning out: If the power circuit in the carburator is plugged with debris you will have perfect idle quality but a terrible hesitation/backfire (this usually happens to either the left or right booster) IF this is the case.. IF! there is an old trick which will pull the debris on through. This whole procedure needs to be done very quickly. Put one hand on the throttle linkage and one hand on the choke air valve. While opening the throttle to near full throttle, quickly close the choke air valve(plate) until it almost dies then release the choke. Repeat this 2 or 3 times until it clears out. Please be careful doing this trick, keep your head out of the way in case of a backfire.
Another possibility
Weak ignition coil: If the coil is weak, only producing 5-10kv(hei), you will have decent idle quality but the coil will break down when more voltage is needed. The bad part about this coil is when its going bad... It acts almost identical to a lean condition. Tough to tell the difference at times.
------------------
Pictures and more...
Email me
'71 LWB
350/350T, PS/PB
Special options? Yes, RUST... Need some?
Garrett
Olathe, Kansas