MG Car Repair: MGB adjusting fuel mixture, adjusting fuel mixture, jet adjustment

Question
Hi Howard,

my question concerns adjusting the fuel mixture on a 1968  MGB with the dual HS4 SU carburetors.  In my 1968 BMC MGB repair manual the technique is to lift the piston in the carb a little, and depending on the response of the engine you turn the needle valve to either lean or enrichen the mixture, on the SAME carb you lifted the piston on, and then do the second carb, back and forth until the mixture is correct.  I have had two local mechanics tell me you are supposed to adjust the needle valve on the OTHER carburetor.  What is the correct procedure?

Oops, I just thought of another, unrelated question.  On my 1968 MGB the choke cable was disconnected, and it looks like I may be missing some hardware.  I have been unable to find a picture of what the assembly at the carburetors looks like; my antique BMC manual does not provide a good picture.  The parts catalogs are not very helpful either.  Might you have any hints?

Thank you in advance.

Answer
Hi Dan,
I know that most books tell you to adjust the carburetor that you lift the pin on. But I was taught to adjust the other carb and I have done so for 40 years. But you are adjusting the jet not the needle valve. A needle valve is a "needle & seat" in the float chamber to control the amount of fuel allowed into the float chamber.
The jet adjustment is the 5/8" nut under the center of each carb.

The choke was not a choke butterfly like on a domestic carb but was a linkage that lowered both jets to richen the mixture for cold start. A short shaft (like the throttle uses) had a lever that the choke cable moved to lower both jets at the same time. The linkage on each carb has a cam that opens the throttle plate (butterfly) on each carb to give it a fast idle during warm-up.

Howard