See Figure 1
The only adjustments covered here are the idle speed and mixture adjustments. These are performed in the course of a normal tune-up. Other carburetor adjustments are covered in Fuel System .
Attempting to reset the mixture on your truck can be difficult and expensive. It is generally not recommended, except on 1967 models without the Cleaner Air Package. No special equipment is required for these models. On all other models, CO meters and propane enrichment systems must be used. These are not only difficult to obtain, but can be very expensive.
Mixture is preset at the factory when the engine is manufactured, and tampering with the mixture screws will usually produce very little change. When performing a tune-up, it is advisable to simply adjust the idle speed screw or solenoid screw, and leave the mixture alone.
Before suspecting the carburetor as the cause of poor performance or rough idle, check the ignition system thoroughly, including the distributor, timing, spark plugs and wires. Also, be sure to check the air cleaner, evaporative emission system, PCV system, EGR valve and engine compression. Check the intake manifold, vacuum hoses and other connections for leaks and cracks.