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1956 Corvette: Service Bulletin: Spark Plug Fouling - Idle Adjustment

Subject:   Spark Plug Fouling - Idle Adjustment
Model and Year:   1956 Corvette - Dual 4-Barrel - Carter Carburetor
Source:  Chevrolet Technical Service Bulletin
Bulletin No:  DR #258
Section:   VI
Date:   August 30, 1956


In cases where Spark Plugs are wet fouling on V-8 Engines used in 1956 Corvettes with dual 4-barrel carburetors being operated in Metropolitan Areas which put considerable limitation on their top speed, it may be necessary to install AC-46 Plugs to correct this condition.

AC-44 Plugs are to be furnished in production.

Range of plug should be tailored to the owner’s requirements:

AC-44  Extreme High Speed Driving

AC-44  Medium High Speed Driving

AC-45  City Driving and Normal Operation

AC-46  Metropolitan City Driving Under Heavy Traffic Conditions


To obtain a consistent idle speed setting on V-8 Engines equipped with Carter 4-Barrel Carburetors with regular or racing camshaft, proceed as follows:

Engine Number Suffix -

GH  Regular Camshaft
GU  Racing Camshaft

  • Set bypass idle screw on each carburetor 1-1/2 to 2 turns open.  (Located on left rear corner of throttle body).
  • Set both idle mixture screws on each carburetor one turn open.
  • Obtain from Parts Stock and install throttle valve set screw #566247, spring #1451460, and washer #566255 on rear carburetor in threaded hole provided.
  • Start engine and set throttle valve set screw for 600 RPM (regular camshaft) or 800-850 RPM (racing camshaft) as required.
  • Adjust all idle mixture screws individually for best engine RPM.
  • Reset throttle valve set screw to original RPM setting (600 or 800-850 RPM).
  • Again adjust all idle mixture screws individually for best engine RPM.
  • Reset throttle valve set screw to 600 or 800-850 RPM.
  • Bypass idle screws on each carburetor need not be adjusted after initial setting in Step #1.
  1. Also 45S plugs are good all around plugs if you have any slight oil consumption problem. The extended tip helps to keep them clean.

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