When rolling about 40 mph in 3rd and you barely increase the throttle it starts stumbling and even loses all power until you back off the throttle a bit, not backfiring.
If I read that right, this will be at 1800 or so rpm, right.
I would bet a dollar you are cruising with the throttle very near to closed.
That would be way down on the transfers, with the Idle mixture screws nearly falling out.
If I am right;
The reason for that would be
a preponderance of low speed torque.
When driving in this mode the engine will pull air around the throttle blades, sometimes drying up the transfers. Then when you ever so slightly tip in the throttles, the transfers got nothing to give. and predictably you get a stumble or stall.
The cure is to give some torque away by retarding the ignition timing. This will cause the engine to drive deeper into the carb, with more throttle opening, keeping the transfers alive.
Disconnect the vacuum advance and roadtest it. If it improves then, I'm probably right. Check your timing, in Neutral, at cruising rpm with the Vcan included. Then at same rpm, recheck it with the V-can disabled. The difference should be 12 to 20 degrees, and the Mechanical, IMO should be 24* or less on a powerful engine with a low-stall TC.
OR
just try it in Second gear at 40 mph, that will require a different throttle opening. If bog-gone, then I am right...... I think..........lol.
