most US webers came with an electric choke. You adjust the choke by loosening the 3 screws on the clamp around the choke housing and turn it so the choke is fully closed when cold. When you go to start the truck, push the petal to the floor once - this should open the choke plate slightly. Start the truck, then tap the petal lightly after 15 seconds to get the choke to step to the next point on the linkage. If it don't step, or drops to base idle, readjust the choke tension and check the linkage until it steps in the proper sequence. Looking at the drawing, those are the primary and secondary jets, but before you change jets, be sure to follow the float setting procedure and make sure the needle and seat assembly is not damaged and seals good. Also check the accelerator pump and check balls to be sure they are working properly.