The difficulties with the FG-20 narrated here caught my interest as I've recently converted a 1.25A to gas with minimal problems using the stock carb. Muffler pressure is used and tuning is tight but not much more so than my Evolution gas engines.
Idle was a bit of a problem but was solved by using a throttle servo speed delay of one second. The delay allowed leaning the idle up enough to get the carb barrel closed to a more normal idle position. Without the delay, idle had to be fat with a somewhat open barrel and of course that restricted it to a fast idle.
Leaning the idle to obtain a smoother and lower idle introduced a spool up stumble that would kill the engine but that was overcome with servo speed slow down. The slow down really doesn't effect flight as spool up happens in about the same time frame, it just prevents a drastic upset of the fuel/air/rpm ratio by opening the barrel and drawing out the metering needle at a speed that gives the rpms enough time to keep up to the changes.
If stumbling is a problem with the FG-20, try a servo speed slowdown.