I think he is referring to the fuel bowl vent mechanism which Volvo referred to as the "hot start valve" (I'm told) on the HIF carbs. To me it makes more sense to call it the fuel bowl vent.

There is a plunger that is activated by a lever on the throttle shaft when at idle or stopped. The plunger opens the valve and thus vents fuel bowl gases to the charcoal canister. There is a small screw that goes through the actuating lever and depresses the plunger. The screw provides an adjustment for how much the plunger is depressed.

FWIW, I haven't had the adjusting screws in my HIFs for years and therefore the fuel bowls haven't vented "properly". I haven't had an issue starting in cold or hot weather.