A close look at the brass damper cap at top of the picture above reveals a small drill hole (little black dot) in the cap for ventilation (vented type damper cap). Also notice the cast rib on outside of the cover just below the cap. The diagram to the right shows an internal drill hole (see arrow) through that rib to vent the top chamber to the lower chamber. Some carburetors have the top vent hole, but not all. Some carburetors have the lower vent hole, but not all. The trick here is that you need exactly one of these holes, but not both.

As the dashpot internal piston rises and falls with carburetor air flow, oil level around the small damper rises and falls past the damper piston. This displaces air in the small top chamber, and that air must be vented somewhere. Early carburetors had a small vent hole drilled in the damper cap. Later design called the "dustless" carburetor had the internal vent hole and a sealed cap. If you have no vent hole the engine will run bad as the piston will rise and fall very slowly making it lean on acceleration (low power) and rich on deceleration (backfire). If you have both vent holes the engine will run really bad as it spoils the chamber vacuum, and the piston may only rise a little or none at all (always lean under power).

So if your engine runs bad, at least once during your ownership of the car you should inspect these parts to assure that the carburetors have exactly one of these vent holes, but not both holes.