[MOTORHEAD]

Got in 2 test drives this afternoon with the 120 holders, 1st drive with idle mix set to 12.5-13 A/F, lean idle. "Punching" it moderately showed mixture going lean with the stumble, and much backfire when backing off the gas. 2nd drive with mix set rich 10-11 A/F, but smoother, Punching it showed mixture also going lean (15-16) but no backfire when off the gas. At this point I would surmise it needs a larger pump shot ? Carbs had blocked pump valves when I got them , but were replaced with .50's. I think I'll try advancing the initial timing first. Zrayr: just read the info you showed, very interesting.
I'll report results as soon as I have some!