There is a slight flat sport low speed idle carb backfire that occasionally occurs. Backfire is a red flag!
What's the ignition timing and curve?
You need to get ignition right before carb changes.
what's the rear gear and tire size?
This only happened 2 twice in a 5 mile test drive and never from a standstill. It happens not always after coasting, make a turn, feather the throttle, carb stumble at 5-7 mph, pop/backfire through the carb and then takes off.
Ignition is the FBO system provided by MRL with FBo ignition box and ballast resistor bypass jumper installed.
The distributor curve was done at MRL. Total was 36* at 3000rpm with the vacuum advance disconnected. I don't have the dyno sheet with me but I believe the initial is 18*
The guy working on finishing it says timing is good and is ASE certified with over 30+ years experience and works on older classics. He is old school. I had a '71 Demon with 318 Rochester that acted the same way with a carb that went away with a Carter. I found out later that was a Chrysler TSB on that carb in '71.
Rear gear was stated up in the thread 3.23 and tires 235/60 R14 rear front 215/70 R14