This was a problem with the early 70's cars, so much so that the manufacturers (GM at least) installed a solenoid (called a dashpot) to operate the throttle blades. When the key was turned off, the dashpot was de-energized and allowed the throttle blades to go completely shut. When the key was turned on, the dashpot was activated and opened the throttle blades to the idle position.

What may be happening in your case is that there may be some carbon in the chambers that is glowing from the heat of combustion which is acting as a glow plug to ignite the little bit of air/fuel mixture that is still flowing past the blades.