I’ve finally been running this motor (muffs) and it starts readily but won’t idle well. It surges badly, about in the range of 700-1500 rev/min,bangs loudly, I could think there was some cross-firing going on.

I’ve been through the carbs and ignition carefully (I thought). The idle mixture is fixed, of course, the throttle flaps are synced, and fully closed at idle. There aren’t a lot of adjustments to make aside from the coordination of throttle and timing.

Can anyone point me in the right direction here?

Thanks.