How to Adjust a Carter Carburetorby Chris Stevenson
The Carter carburetor has been around for a very long time. The WCFB carburetor which came on some of the earliest Corvettes continued in production through the mid-'60s. The Carter AFB model introduced in 1957 began to eclipse the WCFB. The Carter AFB lacked the bulky weight of the WCFB and offered more airflow capacity, dramatically increasing performance parameters. The AFB became popular on Ford, General Motors and Chrysler cars. Today the owner of a Carter carburetor can make some simple adjustments to achieve peak horsepower and performance.
Place the vehicle in "Park" or "Neutral," according to your transmission type. Apply the foot or hand brake. Raise the hood and disconnect the negative battery cable from the battery post. Use a socket to remove the top air cleaner housing bolt, or remove the butterfly nut by hand. Remove the housing. Refer to your Carter manual for the location of the linkage arms that connect to the top case (air horn) of the carburetor.
Remove the hairpin retaining clip that attaches to the choke lever arm, using needle-nose pliers. The choke lever arm operates the the large choke valve in the top of the carburetor. Use pliers to remove the clip that holds the accelerator pump rod to the pump linkage arm -- the accelerator pump plunger sits directly above it. Disconnect the fast idle rod right next to it, in the same fashion. Be certain you know which pin connects which lever or rod.
Loosen and remove all 10 air horn screws on the top of the carburetor, using a screwdriver. Pull the air horn off and tip it upside-down so you can see the float mechanism. The two floats sit on a horizontal plane. Use a small ruler to measure the distance between the gasket surface of the air horn and the bottom of each float. This distance should be exactly 7/16 inch. Use needle-nose pliers to bend the innermost tang (next to the float) to adjust the height. Adjust both float tangs.
Turn the air horn upside down and let the float hang -- this shows the float drop. Use a small ruler to measure the distance from the bottom of the air horn gasket surface to the bottom of each float. The drop distance should be exactly 1-1/4 inch. To adjust the floats, use needle-nose pliers to bend the tangs on the outside of the hinge mechanism, either up or down.
Place the air horn back onto the carburetor throttle bottle and insert the 10 mounting screws. Tighten the screws with a screwdriver. Reconnect the choke lever arm, accelerator pump rod and fast idle rod, in the same fashion you removed them. Use needle-nose pliers to position the clips in place, in the same orientation as when you removed them.
Connect the negative battery cable temporarily by hand. Look at the position of the choke valve at the top of the carburetor. For a cold engine, the valve should be closed. If not closed, loosen the three screws on the circular choke housing and turn the housing dial either clockwise or counterclockwise to open or close the choke valve. Close the valve very slowly by turning the housing dial until it just touches the carburetor throat sides. Tighten the three screws with a screwdriver.
Start the engine and let it warm up to normal operating temperature. Watch to see if the choke valve opens completely. If not, loosen the choke housing screws and make a small turn of the dial to open the valve completely. Re-tighten the choke screws with a screwdriver. Shut the engine off.
Remove the vacuum line at the carburetor base. This line will route to the vacuum advance fitting on the distributor. Connect the end of a vacuum gauge to the vacuum nipple on the carburetor. Locate the idle mixture screws at the base of the carburetor. Use a screwdriver to turn each screw clockwise until they gently seat. Unscrew them counterclockwise 1-1/2 turns for the initial adjustment. Start the engine.
Turn one of the mixture screws clockwise until the engine begins to stumble. Turn the same screw counterclockwise until it reaches the highest vacuum number on the gauge, indicated in inches of mercury. Turn the other idle mixture screw clockwise until the engine begins to stumble. Turn the screw counterclockwise until you reach the highest vacuum reading on the gauge. Shut the engine off.
Connect the negative lead of a tachometer to a bare metal engine source. Connect the other lead of the tachometer to the negative (-) side on the ignition coil. Start the engine. Refer to your owner's manual for the proper idle rpm setting for your engine. Find the idle speed adjustment screw on the throttle linkage cam. It sits above the mixture screws.
Adjust the idle speed screw clockwise or counterclockwise with a screwdriver to set the proper rpm for your engine. For example, your engine might require 700 rpm -- turn the idle speed screw until you reach that number on the tachometer. Shut off the engine.
Remove the vacuum gauge and reconnect the vacuum hose on the carburetor. Disconnect the tachometer leads. Place the air cleaner housing back on the carburetor air horn. Screw the mounting bolt in by hand, or use a socket. Tighten the negative battery cable with a socket. Test run the engine.
- You will have only one float and one idle mixture to adjust on the non-four-barrel carburetor, but it requires the same adjustment procedure. Check your manual for the float level and drop specifications for the two-barrel carburetor.
Things You'll Need
- Carter Carburetor repair manual
- Socket set
- Ratchet wrench
- Needle-nose pliers
- Steel ruler (small)
- Vacuum gauge
Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.