Heater gets warmer during lefthand turns

In my '73 Dart, the heater works relatively 'fine'.
But whenever I take a lefthand turn, the blown air gets noticably warmer for a while.

Has anyone had this same experience and can tell me what might be going on?
I'm thinking there might be a internal heater-door somewhat loose inside, getting 'shut' better at lefthand turns.

(The car came with airconditioning, but that system has been disconnected.)