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Adjusting the Carter BB Carburetors

The BBS and BBD are coming soon to a different page. This article is an overview of how to remove and rebuild a carburetor. Every model is slightly different, but the basic principles apply to all. We are not trying to give specific instructions on how to rebuild every carburetor. If, after reading this, you don't feel comfortable tearing down a carburetor, have a professional or a friend with carburetor experience do the work.

Removing and Rebuilding a Carburetor

This article (except where noted, and items in italics) originally appeared in the March 2011 issue of Skinned Knuckles. Skinned Knuckles is dedicated to the authentic restoration of cars and trucks and to the preservation of vehicles from the brass era through the early 1970s. It is available by subscription only. Articles are copyrighted and all rights reserved. Reprinting authorized by written permission of the publisher only.

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Chrysler Corporation used the Carter BB series carburetor from the late 1940s to the mid 1950s in all product lines. This was the basis for the Carter BBS and BBD series, essentially updated versions of the BB with single or dual barrels.

The original BB was the old Ball & Ball carburetor, as produced by Carter, after they purchased Ball & Ball. Carl Breer himself had helped to develop the BB, working with the Ball family.

The plain-tube, down-draft carburetor has fixed jets which cover all speed ranges except idle — which is controlled by an adjustable needle valve. Turning the needle clockwise gives a leaner mixture and counterclockwise a richer mixture, so that owners could use a vacuum gauge and adjust the carburetor for the highest reading.

Adjusting the BB series

carter logoThe accelerator pump provides the extra fuel which is required for rapid acceleration as the throttle is opened. There are three adjustments on the accelerating pump lever in order to supply the correct amount of fuel depending on weather conditions. For warm weather or altitudes above 914.4 m. (3000 ft.), the pump link should be in the hole in the accelerating pump lever which is nearest the throttle shaft. This is the shortest stroke. For cold weather operation, the pump link should be in the pump lever hole which is farthest from the throttle shaft. For normal summer temperatures, the pump link should be in the center hole.

Carter BB carburetor specifications

Float: Check the carburetor number and make a cardboard or metal gauge. Before adjusting the float, be sure the float lever pin is firmly seated. Bend the vertical lip of the float away from the needle to raise the level, or toward the needle to lower the float level.

Pump: Use Carter Pump Travel Gauge T109-117S. Remove the air horn and back out the throttle adjustment screw. Place the pump operating link in the center hole of the throttle shaft arm. Place the gauge on the edge of the bowl cover so that the lip of the gauge extends over the top of the plunger shaft. Turn the knurled nut of the gauge until the lip contacts the plunger shaft at closed and wide open throttle positions. The difference in the readings obtained at closed and wide open throttle positions should be the number on the gauge given in the chart. This is the plunger travel in 64ths of an inch. Make the adjustment by bending the horizontal portion of the pump lifter link.

If the pump travel gauge is not available, use a scale to measure the distance from wide open to fully closed position and make adjustments as mentioned above.

Fast Idle: Carter BBRemove the thermostatic coil housing, gasket, and baffle plate. With the throttle adjusting screw backed out, open the throttle valve and hold the choke valve fully closed, then close the throttle valve. This will allow the fast idle cam to revolve to the fast idle position (A). There should be the specified clearance (listed in the chart) between the throttle valve and the bore of the carburetor (B). Measure with a wire or flat feeler gauge. Adjust by bending the choke connector rod at lower angle (C).

Unloader: This adjustment must be made after the fast idle adjustment. Hold the throttle valve wide open, and close the choke valve as far as possible without forcing. Use a drill bit of the specified size, and place it at (D) between the upper edge of the choke valve and the inner wall of the air horn. Adjust by bending the arm on the choke trip lever (E).

The automatic choke used with this carburetor is mounted on the exhaust manifold, and a choke rod connects the automatic choke lever to the carburetor choke valve. An electromagnet activated by the starter switch closes the choke valve, and a thermostat takes over after the engine starts. If the engine is hot when being started, the thermostat holds the choke valve open against the action of the electromagnet.

If the choke unit is not operating properly, open the hand throttle and see that the rod from the automatic choke to the carburetor is not bent or binding. Check the choke valve for binding. Check the fast idle rod and cam on the carburetor for excess friction. Clean the linkage and external moving parts with aerosol spray choke cleaner.

Do not lubricate the automatic choke or any of its linkage. There should be an insulating gasket between the exhaust manifold and the automatic choke.

Check the wire from the starter switch to the automatic choke for continuity using a Volt-Ohm Meter. There must also be a good ground connection between the choke and the manifold, and a lock washer between the mounting nut and the automatic choke ensures this connection. The screw that holds the starter switch wire should be just long enough to enter the case. If it is too long, it will hit the choke control and cause an adjustment problem. Check to see that the circuit through the choke unit is complete by holding a screwdriver close to the magnet core while the starter is operated. The screwdriver should be drawn against the magnet core.

Automatic Choke

Remove the air cleaner from the carburetor so you can observe the position of the choke valve. Open the hand throttle approximately one- quarter. Move the automatic choke lever until the hole in the brass shaft lines up with the slot in the bearings, and insert a piece of welding rod through the hole in the shaft. Push the rod all the way down to the engine manifold so it engages in the base of the automatic choke.

Loosen the clamp screw on the automatic choke lever and push the lever upward until the carburetor choke valve is closed tight. Hold the lever in this position, and tighten the clamp screw in the lever. Then remove the wire rod. Replace the air cleaner, checking to be sure that tightening of the air cleaner clamp does not bind the choke valve on the shaft. See that there is no binding in the fast idle or choke mechanism that would interfere with the free operation of the carburetor choke valve. The carburetor throttle should be partly open when making these checks.

The hairpin-shaped thermostat in the base of the choke is not a spring and should not be deformed. It has been set and heat treated, and any changes in its position or shape will throw the automatic choke out of adjustment.


Chrysler products with a semi-automatic (or, later, an automatic) transmission had a “dashpot” to slow the closing of the throttle, prevents a too-rapid return to idle position which could stall the engine.

There is also a kick-down switch on the carburetor, in early models. The purpose of this switch is to bring the car from fourth to third gear when the car is going at a speed where acceleration in third gear is faster than in fourth. The switch is operated by pushing the accelerator pedal to the floorboard.


When the engine is under load, the piston is in the upward position, and the chamber is full of fuel. When the throttle is released and the linkage returns to the idle setting, the piston tends to return to the lower position, and the fuel is discharged through the restricted opening. This restriction retards the closing of the throttle and avoids too rapid a return to idle position which could stall the engine.

The dashpot adjusting screw should be set approximately five full turns out. Further adjustment outward will increase the action of the dashpot and lengthen its retarding effect. Too much retard will affect automatic gear change on the upshift. The transmission has to synchronize before shifting is accomplished, and if the engine speed falls off too slowly when the accelerator is released, gear change will be retarded. The leather seal on the piston controls the retarding action. Should this seal become worn, cracked, or dry, the action will be upset.

When the car speed is below 8 mph in first or second gear and 15 mph in third or fourth gear, the transmission governor points close and energize the dashpot solenoid. This raises the solenoid core and ball check, allowing fuel to enter under the dashpot piston. As the piston tends to return to a lower position, the fuel is discharged through a restricted opening, retarding the closing of the carburetor throttles. When the car speed is greater than the aforementioned speeds, the solenoid is not energized and the ball is down, allowing fuel to be bypassed through openings so that the dashpot does not function.

Kickdown Switch

In operation, the plunger (5) is moved inward when the accelerator is depressed. When the pedal is fully depressed, the plunger makes contact with the arm (3) and moves it against contact (4) which energizes the solenoid circuit, permitting the transmission to function. Contact (4) is attached to the piston (2) which moves up and down, dependent upon the velocity of air at the carburetor venturi.

There are up to four washers placed underneath the spring in the piston, and they should not be changed in service. If the piston and spring are charged, do not use any washers. If only the spring is to be replaced, do not add or remove any washers from the piston. To check if the switch is working, do a continuity test between the hot terminal on the voltage regulator and the terminal on the carburetor. To check the switch, if the switch is functioning, drive the car in second gear and accelerate rapidly from a speed of about 20 mph. The kick-down should occur between 22 to 25 mph.

Orest Lazarowich’s writing originally appeared in the March 2011 issue of Skinned Knuckles. It is copyrighted by SK Publishing and may not be reproduced, in whole or in part, without written permission from SK Publishing. See Skinned Knuckles for more vintage and classic car tips. Also see vintage car repairs and these other articles from Skinned Knuckles:

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