How do you adjust a Model T carburetor? It is very simple. Initial adjustment is as follows: Screw the idling jet (lower knob/screw)all the way in, then back out 1 turn. Screw the high speed idle jet (higher knob/screw)in all the way, then back out 2-1/2 turns. This will get the car started. When do I need to adjust the carburetor in my car? If your engine is running too rough, it's important to adjust the mixture and find the correct idling speed to decrease engine stress, keeping things from running too fast or too slowly. Adjusting the carburetor in...