All carburetors have tamper-resistant idle mixture adjusting screws. This setting has been performed at the factory. Neither removal of the plug or adjustment of the mixture screw is required during service unless a major carburetor overhaul is performed.
- Remove the carburetor from the engine as described in Fuel System . The idle mixture screw is located in the base of the carburetor, just to the left of the PCV hose. Mount the carburetor, carefully, in a soft-jawed vise, protecting the gasket surface, and with the mixture adjusting screw facing upward.
- Drill a 5 / 64 in. (2 mm) hole through the casting from the underside of the carburetor. Make sure that this hole intersects the passage leading to the mixture adjustment screw just behind the plug. Now, widen that hole with a 1 / 8 in. (3.18 mm) drill bit.
- Insert a blunt punch into the hole and tap out the plug. Install the carburetor on the engine and connect all hoses, lines, etc.
- Start the engine and run it at fast idle until it reaches normal operating temperature. Make sure that all accessories are OFF and the transmission is in Neutral (manual) or Park (automatic). Turn the ignition switch OFF and disconnect the battery ground cable for about 5 seconds, then reconnect it. Disconnect the oxygen sensor.
- Start the engine and run it for at least 5 seconds at 2,000-3,000 rpm. Then allow the engine to idle for about 2 minutes.
- Connect a tachometer and allow the engine to operate at the curb idle speed. Adjust it, if necessary. Insert a CO meter probe into the exhaust pipe. A reading of 0.1-0.3% is necessary. Adjust the mixture screw to obtain the reading. If, during this adjustment, the idle speed is varied more than 100 rpm in either direction, reset the idle speed and readjust the CO until both specifications are met simultaneously. Shut off the engine, reconnect the oxygen sensor and install a new concealment plug.
FUEL INJECTED ENGINES
Fuel injected trucks use a electronic fuel injection system which is controlled by a series of temperature, altitude and air flow sensors which feed information into a Electronic Control Unit (ECU). The control unit then relays an electronic signal to the injector nozzle(s), which allows a predetermined amount of fuel into the combustion chamber.