Most carburetors that use needle valves are the Rochester Quadrajet, Edlebrock and Carter AFB carburetors. The needle jet is a long thin rod tapered at the bottom that is placed in the inlet side of the main jets. The rod responds to the movement of the throttle. As it is lifted out of the main jet, more fuel is allowed to flow; conversely, as it is dropped into the jet, less fuel flows.
Remove all the Phillips screws from the top of the carburetor using the screwdriver. Push the roll pin out of the accelerator arm pivot at the top of the carburetor, using the scribe. This is the arm that depresses the accelerator pump when the throttle is opened. Lift the arm out of the support fulcrum and lay it aside still connected to the throttle linkage.
Disconnect the rod to the choke plate by removing the retaining clip on the base of the rod where it is attached to the actuator arm, using a scribe. Do not use these super small retaining clips as they are removed.
Lift the top of the carburetor off the intake--carefully--so the gasket does not tear. The two long thin rods with tapered ends suspended in the center of the top carburetor cover are the main metering rods or needle valves. Remove the two main metering jets on the bottom of the float bowl, using the common screwdriver.
Spray carburetor cleaner through the holes from which the main jets were extracted. Use the tube on the carburetor cleaner can so direct pressure can be applied to the holes, which will blow out the entire passage. Clean the main jets with the carburetor cleaner and allow them to dry.
Clean the needle valves with cleaner and inspect them for any imperfections. The main metering rods or needle valves will not function properly if they are bent or broken on the ends and must be replaced.
Lift the top of the carburetor--complete with the metering rods--up and center it over the carburetor body. Lower it slowly, using the scribe to align the rods with the main metering jets. As the top is lowered, if the rods contact the jets do not allow the pressure of the weight of the top rest on them. Lift the top up slightly and realign the rods until they fit into the metering jets. The top should drop all the way down with no feeling of binding when the rods enter the main jets properly. This is a delicate and frustrating job at times, so be patient and precise.
Install all the screws in the top of the carburetor and tighten with the Phillips screwdriver. Push the choke rod into the activating lever and insert the retaining clip.
Lift the accelerator lever up, swing it around so it lines up with the accelerator pump push rod, lower the lever into the support arm, line up the hole in the arm and the lever and insert the roll pin. Push it the rest of the way in with the scribe.
