MG Car Club

It's all about the MG's - The British Sports Car America Loved First

Log in

Tuning SU Carburetors for an MGA

September 16, 2018 2:23 AM | Bob Chalker (Administrator)

By  Don Lantz

Start by removing both air filters, and (brass or black plastic) dash pot covers. Check to make sure that enough oil is in the piston dampers on both carbs (no higher than the top of the smaller part of the inside bore of the inner moving piston. Also, feel the bottom of the carburetors to check if it is damp indicating a bit of fuel leakage.

From inside the car, pull out the choke full open and release it. The choke on both carburetors should be closed. Check by reaching under each carb and pressing up to check if the jets have returned to the full up rest position. In the process, if you noticed a jet is not up to the rest position, then check that the jet tension spring is attached to the jet lever and carburetor body.


1)     Start the engine and run it up to normal running temperature. Loosen one of the screws on an accordion (zig-zag looking) clamp on the throttle shaft between the carbs so that the carbs can be adjusted one at a time.

2)     With the engine running, turn the idle screw on one of the carbs back (counter clockwise) until its lever is resting on the top of the throttle stop and the throttle disc is completely closed.  This will isolate that carb so the other carb can be adjusted and tuned. The engine will not stop running as there is a balance tube on the intake manifold connecting the two carbs so that all the cylinders will continue run on the fuel air mix from the one carb.

3)     Now, set the idle just below 1,000 RPMs using the idle screw of the operating carb.

4)     Keep the idle adjusted to stay below 1,000 RPMs during the tuning process.

5)     Turn the jet adjusting (mixture nut) clock wise when looking down from above (this lowers the jet in the bridge area of the carburetor) which allows more fuel to be exposed to the incoming air, causing the fuel air ratio to be richer.

6)     Continue turning the jet nut downward until the car engine makes a galloping like sound at idle.

7)     Then turn the jet nut back slowly towards lean until the engine begins to shake and may have a slight misfire indicating the fuel air mixture is too lean.

8)     Next, turn the jet adjusting nut back towards the rich directions until the engine shake just goes away.

9)     Then turn the jet adjusting nut two more flats towards rich. As the engine smooths out, the idle speed will increase slightly.

10)   Reduce the idle speed back down to under 1,000 RPMs.

11)   That carburetor’s mixture adjustment is now complete.

12)   Close the throttle on that carb and open the second carb’s throttle. 

13)  Repeat steps number 5 through 11 on the second carb.


1)     After completing the second carb’s mixture adjustment, the carbs need to be synchronized. This can be done with or without special equipment.

2)     We will synchronize without any special equipment.

3)     Carefully determine when the throttle arm and its adjustment screw contact the throttle stop at the same time. Do this by slowly backing the idle screw out while placing a finger on the idle arm. When the idle arm stops moving, turn the idle screw back and forth several times to be certain of when the idle arm is just resting on the idle stop. Now turn idle screw ½ turn clockwise to open the throttle for slow idle on that carb.

4)     Do the exact same procedure for the other carb.

5)     Tighten the previously loosened screw on the accordion clamp to lock together the two synchronized carbs.

6)     Adjust the engine idle to just below 1,000 RPMs by turning both idle screws the same amount in the same direction.

7)     Any time the idle needs adjusting, adjust both carb’s idle screw the same amount of turn, and in the same direction.

8)     You can check for proper adjustment by reaching under the rim on the right side of each carb pressing upward on the piston lifting pin. This pin raises the air piston about 1/32”. If the engine stumbles, the mixture is too lean; if the engine speeds up and runs faster, the mixture is too rich; if the engine speeds up a small amount and then slows back to the original speed, the mixture is well adjusted. Note that the difference between being too rich and too lean is about one flat turn of the jet adjusting nut.


Next is the adjustment of the linkage connection between the two carbs. This can be very important especially in cold climate areas as they allow for more fuel to enter the carbs for richer start up and running till the engine reaches normal operating temperature.

1)     Remove the slack out of the choke linkage at the interconnector link by loosening the top and bottom nuts.

2)     Tighten the lower nut on the interconnector so that there is a slight bit of pressure on the lever. Do not over compensate as it preloads the choke causing a richer mixture.

3)     Tighten down the top nut to lock the adjustment.

4)     Hook up the choke cable by giving it a slight twist and a little slack before locking it down.

5)     Connect the choke cam link to the #2 hole on the cam for our climate area and secure it.

Contact Us:  HoustonMGCarClub@gmail.com

Houston MG Car Club

PO Box 40711

Houston, TX   77240. 

Powered by Wild Apricot Membership Software