Are you sure the vacuum tank is working? Did you add fuel to vacuum tank to get it started? It will run for a short distance on a manual prime. If vac tank isn't pulling fuel it will soon go lea, run out of gas. If adding another prime to tank gets it going again then the tank isn't working. If it is truly flooding it could be Jon's scenario or perhaps a carb float or needle/seat problem. I had to get a new float for mine - it had small cracks in the brass and was filling with fuel. Luckily, repro floats are available.