Check your fuel tank and its contents : drain half a litre or so from the fuel hose at the carb into a large clean glass jar (apply a "vacuum" to the vacuum hose that's connected to the intake manifold, otherwise it won't flow...) and check the following :
-does the fuel filter allow for a continuous flow ? (an A50 needs 1..2 litres/hour)
-is there any dirt coming through the filter ? (even the tiniest bit is a problem)
-is there any water in the gasoline ? (water will separate and sink to the bottom)
Also : how old is the gasoline in the tank anyway ? Gasoline isn't what it used to be anymore nowadays... If you didn't refuel very often or left it standing for more than a month, it's probably better to just drain it completely and burn the old gasoline in your cage, or give it to someone who drives one - modern cages have engine management systems and fuel systems that can deal a lot better with deteriorated ethanol contaminated gasoline than our 1960s technology does...