OEM quality idle mixture screws and springs for automotive and marine Rochester Quadrajet 4 barrel carburetors. Unfortunately there is no data that can match the idle mixture screw to a specific carburetor number, so the idle screws are listed by size. If you still have your old idle mixture screw, compare it to our new screws. Except for the later metric screws,(metric screw heads are of the ?double D? design) all other screws are 10-32 thread. The difference in the screws are length, tip taper and head design. An idle screw with a longer taper will give you more ?feel? when adjusting the idle air/fuel ratio than a short taper screw.