Article 937 - Lean Carburetion (Surge) at Constant Speed - 40-60 MPH

TSB #50 - November 4, 1966
(SUPERSEDES SPECIAL SERVICE LETTER #37 OF SEPT. 16, 1966)
(1967 390-2V Non-Thermactor Automatic Transmission Units Built Prior to Approximately December 1, 1966)


Early investigation has revealed that a lean fuel condition may exist at constant speed in the 40-60 MPH range on subject 390-2V non-Thermactor units with automatic transmission. Although vehicle speed remains constant, the customer may notice slight engine speed fluctuation.

The following corrective action was taken in production to avoid customer complaints of this problem:
    1. The carburetor main metering jets were enlarged one size to size number 54F.
    2. A revised gasket was installed between the intake manifold and carburetor heat spacer.
    3. The distributor vacuum and centrifugal advance curves were modified.
These three modifications are all required to assure problem resolution on subject units; however, the modifications were not applied to production units simultaneously. It is therefore possible to encounter the subject customer complaint on a unit which contains two of these three required modifications. A means of identifying each of these modifications on a given problem unit is included in the procedure to avoid unnecessary corrective efforts.

Necessary parts to perform these three modifications have been released in a service kit. (Part Number C7AZ-9D544-A, Class "A"). All parts included in this kit may not be required on a given problem unit.

1. CARBURETOR MODIFICATION -Subject units equipped with carburetor models C7AF-BR or C70F-AK contain this modification. Install the main metering jets supplied in this service kit (54F) and check the wet fuel level specification (25/32 inch) to revise previous carburetor models C7AF-AK or C70F-U.
2. INTAKE MANIFOLD TO CARBURETOR HEAT SPACER GASKET -The revised gasket has a large tab which protrudes from the rear of the carburetor heat spacer. (Refer to the service kit gasket to identify this tab). If this tab is not visible on a complaint unit, a revised gasket must be installed.
3. DISTRIBUTOR MODIFICATION -Distributor model C7AF-AG has been
modified. The following procedure must be followed to modify previous distributor models C7AF-Y and C7OF-AB to this latest specification:
    A. Remove the distributor and install in a test stand.
    B. Remove the distributor vacuum diaphragm nut and discard the spring and phenolic stop. (Illus. S1138)
    C. Install the red advance spring and revised phenolic stop (.943 - .947 inch) supplied in the service kit. Add a sufficient number of spacers and reinstall the diaphragm nut. (25 - 30 ft. lb.)
    D. Recalibrate both the vacuum and centrifugal advance portions of the distributor to comply with the revised distributor specifications:
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    E. Install the revised distributor.
4. A. AIR CLEANER - The proper 390-2V air cleaner has a baffle between the air cleaner element and air inlet tube. (Illus. S1140) An air cleaner without this baffle will contribute to the subject problem and must be replaced.
B. VACUUM SYSTEM - Inspect the vacuum system for leakage. Any engine vacuum leak must be repaired to assure problem correction.
5. Set the ignition timing to the specified 12 degrees BTDC (distributor vacuum line disconnected) and adjust the carburetor mixture and idle speed to complete the corrective procedure.

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Illus. S1138
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Illus. S1140


WARRANTY STATUS - REIMBURSABLE
Oper: Use applicable suggested time schedule operation.

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