I’d start with engine off, remove air cleaner, look down the throats and open the throttle. You should see a healthy squirt of gas in each throat. If not, you need a new accelerator pump. I cant remember if this one uses the plunger or the rubber diaphragm and lever. I think its the latter. When you open the throttle, the lever should push into the square cover on thhe side. Remove the cover and replace the rubber diaphragm underneath.
If thats not it, check for the smell of gas after it as run for a minute. If htere is a strong odor, I’d replace the needle and seat and set the float about a 1/4" lower than specified. It helps to prevent overflow on hard cornering.
BTW, lest there be any confusion, when you step on the gas, the vacuum advance retards the timing. Step on gas, LESS vacuum, LESS vacuum advance. There is also a mechanical advance in the distributor that use weights and springs. A little light oil on the hinge poinds and slides helps this to work smoothly.