There is a disc/mushroom shaped thing with a vacuum pipe connected to it on the housing where the hot air and cold air pipes join onto the air filter housing. This is a vacuum operated device that when the air temp is cold, a temperature sensitive valve directs vacuum to this "actuator" which pulls on a flap, closing the route for cold air and allowing only the hot air in to stop the carb from "icing".
If you link the two vacuum pipes from the temp sensitive switch together with the engine running, the flap should move to close off the cold side of the air inlet to the filter.
Hope this makes sense, been a long day

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Steve.