In late 60's and 70's, there was a thermal valve mounted on the bottom or side of the air cleaner housing. As temperatures rose the bimetal valve would allow manifold vacuum to flow to the air gates located at the end of the snorkels. These air cleaners used an air tube connected to one or the other rocker cover via an oil separator type cap, and the vacuum valve was located in close proximity. As warm air came up from the bowels of the engine (being sucked into the housing by negative pressure due to carb operation) the vacuum valve would cycle the gates from closed to wide open progressively according to the air temperature being sucked in and blended with incoming air via the snorkels. Some vacuum valves were not that sensitive and were either open or closed with no progression. All valves were highly temperature sensitive.
