Carburettor icing can happen in any carb if the atmospheric conditions are correct. It occurs when the air outside is humid. The venturi effect of the carb can drop the temperature significantly. For example, although it might be a muggy 18 degrees outside, the air pressure inside the carb means it's 1-2 degrees and capable of freezing moisture. This can then cause ice to form on the surface of the carb's throat (and also the throttle valve). In the short term, it increases the venturi effect but ultimately airflow will be restricted and the engine will run too rich. If you're still struggling to get your head around how humid air can 'freeze', lick your finger and notice how much colder it feels when you blow on it. The only real fix carb icing is to install heaters, but this isn't really possible in most circumstances. Things can be improved by using a premium fuel or an additive, while some cars have an option to reroute warm air in the direction of the carb. That said, air-cooled VWs do normally have an inlet air pre-warmer or a heat riser under the intake manifold on the carb itself. Check if you have these and that they're working correctly.