If it were me, I would check 2 things.

1) Max (total) spark advance. You need a timing light with adjustable advance, but total timing could be an issue. Total timing should be around 36 to 38, right?

2) Try adding fuel under load. If the engine will hesitate on a snap throttle, do it that way. If not, add extra fuel via a vacuum line and propane or carb spray. I have had issues with carb emulsion tubes plug up with debris that looked fine on inspection.

You want to try to give the engine what it wants. It acts like it wants fuel (lean backfire). Give it some and see how that works. Note: do not use ether (starting fluid)! You need something that will burn similar to gas.

If you do find adding fuel helps, it means you have too little fuel metered for the correct amount of air ingested, or too much air for the right amount of fuel metered.

Mark