See Figure 1
- Remove the carburetor.
- Remove the float bowl cover and invert the bowl. Hold the retaining spring in place.
- Place the gasket on the float bowl.
- Place a straightedge across the surface of the bowl. It should just touch the floats +-1 / 32 ".
- If adjustment is necessary, bend the float tang to obtain the correct adjustment.
See Figure 2
- Place the throttle linkage in the curb idle position.
- Place the pump operating link in the middle slot on model R-7847-A, or in the left slot on all others.
- With a T-scale, measure the length of the pump operating link. On models through 1972, the link should be 2 7 / 32 " on R-7847-A models; 2 21 / 64 " on all other models. On 1973-79 models, the link should be 2 7 / 32 " for trucks with manual transmission, or 2 11 / 32 " for trucks with automatic transmission. On 1980 and later trucks, the link should be 1 5 / 8 ". Bend the link at the angle, to adjust.
See Figure 3
- 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 0.080" drill bit between the upper edge of the choke plate and the wall of the air horn.
- 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 4
- Hold the throttle valve in the wide open position.
- Insert a 1 / 4 " drill bit between the upper edge of the choke valve and the wall of the air horn.
- With a finger pressing lightly against the choke plate, a slight drag should be felt as the drill is being withdrawn.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 5
- 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.
- On models through 1972, insert a 0.110" drill bit between the choke valve and the wall of the air horn. On 1973-77 models, use a 0.140" drill bit for manual transmission trucks or 0.090" for automatic transmission trucks. On 1978-79 models, use a 0.100" drill bit. On 1980 and later models, use a 0.130" 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 6
- Run the engine to normal operating temperature.
- With the engine off and the transmission in Park or Neutral, open the throttle slightly. Remove the air cleaner and disconnect the vacuum lines to the heated air control and the OSAC (Orifice Spark Advance Control) valve. If there is no OSAC valve, disconnect the distributor vacuum advance line. Disconnect the EGR hose. Cap all carburetor vacuum fittings.
- Close the choke plate until the fast idle screw can be positioned on the second highest speed step of the cam.
- Start the engine.
- Turn the fast idle speed screw in or out until the speed shown on your truck's underhood sticker is obtained.
- Stopping the engine between adjustments is not necessary. However, be sure to position the fast idle speed screw on the cam after each speed adjustment.
See Figure 7
- Set the curb idle and mixture.
- Start the engine and position the throttle lever so that the actuating tab on the lever is contacting the stem of the dashpot [cf2]without depressing it!
- Allow about 30 seconds for the engine speed to stabilize. The engine speed should be 2,500 rpm.
- If not, loosen the locknut and turn the dashpot in or out as necessary.
- Hold the dashpot and tighten the locknut.