Article 933 - Rich and Erratic Engine Idle

TSB #50 - November 4, 1966
(All 1967 Units with 4300 Design Carburetor - 289-4V, 390-4V non-GT, 428-4V)


Customer complaints of rich and erratic engine idle may be caused by a binding secondary throttle shaft which will not allow the secondary throttle plates to fully close. The following procedure will verify this problem cause:
    1. With the throttle at idle, rock each end of the secondary throttle shaft up and down. If definite throttle shaft to bore clearance can be detected at each end, the throttle shaft is not the cause of this customer complaint. If up and down movement is not readily apparent at each end, continue with diagnostic step #2.
    2. Actuate the secondary throttle plates by holding the choke plate open (vertical) and fully open the primary throttle plates. Be sure the secondary throttle shaft return spring is properly attached. Slowly close the primary throttle plates and observe the return of the secondary throttle.

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Illus. S1133
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Illus. S1134


If the secondary throttle does not fully close, a tight or bent throttle shaft is confirmed. Use the following corrective procedure:
    1. Lightly tap the top of the secondary throttle shaft on the choke side of the carburetor using a 1/4 to 1/2 inch diameter rod and light hammer. (Illus. S1133).
    2. Similarly, tap the top of the secondary throttle shaft on the throttle linkage side of the carburetor (Illus. S1134) until vertical shaft movement is apparent.
    3. Start the engine with a tachometer connected. With the engine at idle speed, slightly rotate (open) the secondary throttle shaft and allow it to close normally. If the engine repeatedly re-returns to a constant smooth idle, this problem is resolved. If the secondary throttle plates do not always fully close, the throttle shaft must be replaced in accordance with proper maintenance manual procedure.

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