The vaporisation usually comes about because the fuel supply to the carb is marginal due to partial obstruction or wear to the pump components.In normal traffic conditions my a35 runs ok. but in slow moving traffic and queues (getting into shows) it stalls. Two goes on the starter and it starts. Five minutes later it does the same. When traffic returns to normal it runs ok.
Examine the pump and check that it's working as it should and that the filter is clear. Renew or recondition as necessary.
Disconnect the fuel line at both ends and blow through it with an airline to ensure it is clear. If the pipe is the original steel one, check that it isn't holed by corrosion, allowing it to pull air in (usually towards the rear of the run).
The tanks originally have an internal filter that's impossible to service and the cars were obviously never intended to last 50 years. It can be disabled by poking a rod up the fuel outlet and forcing it through the gauze. I recommend you use brass as a steel rod could cause a spark.
Install a new inline filter in the pipe near to the tank.
If you've done all of the above and still have the problem, some people find that routing the pipe that runs between the pump and carb further away from the hot exhaust manifold can help.
Check the colour of the spark plugs, they should be a straw colour. If they are lighter or white even, the mixture is too weak. Zenith carburettors are renowned for running weak. The carb body distorts where it joins the manifold and also where the float chamber attaches, allowing air to be pulled past the gaskets.