OK, good.
I would suggest you reattach the vacuum gauge and put it where you can can watch it from the cab when you drive.
1. lean mixture
Accelerator pump not working
Main jet too small or damaged/dirty
Float set too low
Vacuum leak (appears you ruled that out)
2. Timing
Forget about timing marks - set the timing with the vacuum gauge.
Vacuum advance diaphragm bad (you checked that)
unusual problems-
Mechanical advance sticking, bad/missing springs
Vacuum advance sticking
3. crossed wires
I know it sounds insulting, But I once had to look at a Mustang running in SCCA events. It was a almost impossible to start but once running it was finishing in the top 5. I found 2 spark plugs wires were crossed. Swapped them and it started fine and finished 2nd.
4. Sticking valves (or too tight)
The vacuum gauge will show this if it is intermittent (of course the backfire will screw it up for a second)or comes on when hot.
5. spark occurring at the wrong time
Cross fire can happen when spark plug wires with weak insulation are running parallel (in a bundle)and the spark can jump to the wrong plug.
Cracked or dirty distributor cap arcing to the wrong plug.
Actually found a tractor engine that had so much carbon in the combustion chamber that when it was hot the fuel mixture would ignite on the intake stroke from hot spots on the carbon and backfire into the manifold.
