Absolutely correct- foam air cleaners are dangerous! What happens is, reversion causes a small cloud of fuel to hang above the carb throats and the foam air cleaners soaks it up. Then the slightest backfire ignites the fuel up in the air cleaner. Could be a very slightly burned valve, too much timing, a big cam or just a cold engine. And it's worse on Weber carbs because the reversion pulses are stronger so the reversion cloud is bigger, with more fuel. Webers do not have chokes as such- they have an 'enrichment' circuit instead.
When- not if- the gas-soaked foam ignites, the tall pot-metal aux venturis in Weber carbs melts. Molten metal runs down the short intake stacks & into the combustion chambers where the intake valves are open. The NEXT time the piston comes up in that cylinder, the molten pot-metal inside has solidified and you break a piston. Happened to me and others, even in warm sunny California. Also was common on 911 Porsches in the '70s. That's why I started carrying a fire extinguisher in all our cars.