The Marvel Schebler choke is used during cold starting and during the warm up period. When the engine is cold it needs an additional fuel/air mixture.
The Marvel Schebler carburetor has a choke valve (25) located on the carburetor air inlet and is manually operating by the driver. When closing the choke valve air is restricted from entering the carburetor and at the same time increases the suction from the nozzle (14). Consequently you get a higher, or richer mixture of gas.
As soon as the engine starts up, the user will need to start closing the choke, or the engine will choke out. The valve only needs to move slightly. At the same time the spring loaded relief valve will open automatically with engine speed. This eliminates most of the manipulating of the choke when 1st starting up. If the relief valve, or the spring over the valve is missing on your carburetor you may experience a harder start when 1st starting up. The missing valve will allow too much air into the carburetor. A frozen valve will not allow the valve to open when accelerating a cold engine possibly causing the engine to die after starting.
Once the engine reaches normal operating temperature, the choke valve should be completely open, or an excessive amount of unburned gas is going to be sucked down into the engine causing excessive engine wear.
Make any adjustment except to the float when the engine is at operating temperature and the choke is wide open.