I'm guessing the secondary air door needs some tuning...reacts to air flow..
check this. from
http://australianfordhardtopclub.org/Carter_Thermoquad_info.htmSPRING ADJUSTMENT:
Eric can supply an invaluable tool for adjusting the secondary air-door
The air-door spring is located inside the carburettor, and is wrapped around the air-door shaft. Looking at the left side of the carburettor, the end of the air-door shaft is visible almost flush with the surface of the carb top cover. The shaft appears to ride in a slotted sleeve, and the end of the shaft itself is slotted. Spring tension is adjusted by turning the shaft itself. The slotted sleeve is actually the shaft lock. To adjust the spring tension, the lock is released by rotating it anti-clockwise, spring tension is then increased by rotating the shaft ANTI-clockwise, or decreased by rotating the shaft CLOCKWISE. When the desired tension is achieved, the shaft is locked by tightening the lock in a clockwise direction. Being spring loaded, of course, the shaft will attempt to spin itself fully clockwise whenever the lock is released. It is therefore necessary to hold the shaft in it's current position whenever the lock is released, or in the process of being locked or released. It's easiest to make these adjustments by using a screwdriver on the shaft, and the factory tool (or a copy of it) to release and tighten the lock whilst the screwdriver holds the shaft in place. It's difficult, but not impossible to just use two screwdrivers