Carburetor idle mixture is preset at the factory and a plastic limiter cap is mounted on the idle mixture screw. The cap limits the mixture screw to approximately one turn leaner (clockwise) without breaking the cap. Idle mixture should be adjusted at major carburetor overhaul. 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.
On 1978-80 models, a change was made on some General Motors carburetors to limit the range of idle mixture adjustment on the rich side. In other words, backing out the adjustment screw will not make an appreciable difference. The new procedure requires the use of artificial enrichment through the addition of propane. Since it is not feasible to buy a tank of propane for one or two carburetor adjustments, the procedure for cars from 1978-80 are not covered here. On 1981 and later models, the idle mixture is adjusted by the CCC system and requires no manual adjustment.1976-77 VEHICLES
- With the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, attach a tachometer to the engine. Apply parking brake, block the rear wheels, and disconnect and plug the PCV hose at vapor canister and vacuum advance hose at the distributor.
- Start the engine and check ignition timing. Adjust the timing as necessary. Replace vacuum advance hose.
- Place automatic transmission in Drive or manual transmission in Neutral.
If the mixture screw is removed from the carburetor, gently seat it, then back it out 3 turns. Continue with step 4.
- Remove the air cleaner, cut the tab off the limiter cap and remove the cap from the screw. Replace the air cleaner. Obtain the maximum idle speed by turning the mixture screw clockwise (leaner) or counterclockwise (richer).
- Turn the idle speed solenoid in or out to obtain the higher idle speed stated on the underhood tune-up specifications sticker.
- While turning the idle mixture screw clockwise (leaner), watch the tachometer to obtain the lower idle speed stated on the underhood specifications sticker.
- Stop engine, remove the tachometer, and replace the PCV and vacuum advance hoses.