See Figure 1
- Position the fast idle speed adjusting screw so that it contacts the second highest step of the cam. Move the choke valve toward the closed position with light pressure on the choke linkage.
- Insert a 0.070" drill bit between the upper edge of the choke plate and the wall of the air horn. On carb. #40172-A, use a 0.052" drill bit.
- If adjustment is necessary, bend the fast idle link at the angle until the specified drill fits between the choke plate and the wall of the air horn.
See Figure 2
- If the adjustment is to be made on the engine (with the engine running at curb idle), back off the fast idle screw until the choke can be closed to the kick position. Note the number of screw turns required so that the fast idle can be returned to the original adjustment.
- If the adjustment is to be made off the engine, open the throttle valve and move the choke to its closed position. Release the throttle first and then release the choke. Disconnect the vacuum hose from the carburetor body and apply a vacuum of at least 15 in.Hg.
- For models R-8999A and R-9000A, insert a 0.130" drill bit between the choke valve and the wall of the air horn. On all other models, use a 0.150" drill bit.
- Apply sufficient closing pressure to the choke lever to provide a minimum valve opening without distorting the diaphragm link (which connects the choke lever to the vacuum diaphragm). Note that the cylindrical stem of the diaphragm will extend as its internal spring is compressed. This spring must be fully compressed for the proper measurement of the vacuum kick adjustment.
- Remove the drill bit. If a slight drag is not felt as the drill or gauge is removed, an adjustment of the diaphragm link is necessary to obtain the proper clearance. Shorten or lengthen the diaphragm link by carefully closing or opening the U-bend in the link until the correct adjustment is obtained.
- Refit the vacuum hose to the carburetor body (if it had been removed) and return the fast idle screw to its original location.
- With no vacuum applied to the diaphragm, the choke valve should move freely between its open and closed positions. If it does not move freely, examine the linkage for misalignment or interference which may have been caused by the bending operation. If necessary, repeat the adjustment to provide the proper link operation.
See Figure 3
- Hold the throttle valve in the wide open position.
- On 1980-81 trucks, insert a 0.310" drill bit between the upper edge of the choke valve and the wall of the air horn. On 1982-84 models, use a 0.200" drill bit. On 1985-87 models the drill bit is 0.150" for all except carb. #40172-A; that carb. is 0.200".
- With a finger pressing lightly against the choke plate, a slight drag should be felt as the drill is being withdrawn.
- If adjustment is necessary, bend the unloader tang on the throttle lever until the correct opening is obtained.
See Figure 4
- Remove the bowl vent cover plate, vent valve lever spring and retainer. Remove the vent valve lever and pivot pin.
- Hold the throttle in the wide open position.
- Insert a 5 / 64 " Allen wrench in the power valve adjustment screw.
- Push the screw down and release it to determine if clearance exists. Turn the screw clockwise until clearance is zero.
- Turn the screw 1 turn counterclockwise.
- Install the removed parts.
See Figure 5
- Remove the bowl vent cover plate and vent valve lever spring. Be careful not to dislodge or lose the vent valve retainer.
- Make sure that the pump connector rod is in the inner slot of the pump arm.
- Position the throttle at curb idle.
- Place a straightedge on the bowl vent cover surface of the air horn over the accelerator pump connector rod in the center of the bowl.
- Bend the accelerator pump connector rod until the lever surface is flush with the air horn surface on models through 1985. On 1986-87 models, set the float for a 0.050" gap.
- Install the vent valve lever spring and bowl vent cover plate.
If this adjustment is changed, both the bowl vent and the mechanical power valve adjustment must be reset.Fast Idle Speed
See Figure 6
- Run the engine to normal operating temperature.
- Remove the air cleaner and disconnect and plug the EGR vacuum line and distributor advance vacuum line. If the engine has Electronic Spark Control, remove only the top of the air cleaner housing and lift the air cleaner to access the carburetor. Do not disconnect the vacuum line to the spark control computer. Instead, use a jumper wire to ground the idle stop switch.
- Set the parking brake and place the transmission in Neutral. With the engine off, open the throttle, close the choke and close the throttle.
- Rotate the fast idle cam until the fast idle screw can be positioned on the second highest speed step of the cam.
- Start the engine. Let the engine speed stabilize. If the engine speed continues to rise, the idle switch has not been properly grounded.
- Turn the fast idle speed screw in or out until the engine speed listed on your truck's underhood sticker is reached.
- Reposition the screw on the cam after each adjustment to provide the correct closing torque.
- Remove the jumper wire, reconnect the vacuum hoses and install the air cleaner.
See Figure 7
- Remove the carburetor.
- Remove the air horn.
- Invert the carburetor body. THE PUMP INTAKE CHECK BALL WILL DROP OUT. BE SURE TO CATCH IT!
- Allow the weight of the floats to rest on the needle valve. Hold your finger against the hinge pin retainer to fully seat it in the float pin cradle.
- Using a straightedge, measure the gap between surface of the float bowl to the toe of each float. The gap should be 9 / 32 " +-1 / 32 ".
- If adjustment is necessary, bend the float tang. Bend either float arm to equalize the individual float positions.
- Remove the bowl vent valve cover plate and the vent lever spring.
- With the throttle in the curb idle position, press down firmly on the vent valve lever where the spring seats. Simultaneously, press down on the vent valve tang until the vent valve is lightly seated.
If this adjustment is being performed off-engine, the carburetor must be on a raised flat surface so that the curb idle throttle plate position is not changed.
- While holding these things down, measure the gap between the contact surfaces of the vent valve lever and the vent valve tang. The gap should be 0.030".
- If not, adjust by bending the end of the vent valve lever.
- Install the lever spring and cover.
If this adjustment is changed, the accelerator pump adjustment must be reset.