HD-8 SU Carburetors

By Tom Mason

Originally published in the Austin-Healey Magazine, December 1991


Well, if there's a more controversial subject than SU carbs, I don't know what that would be. The main problem with SU carbs is Lucas electrics. So before you touch a carb, it is absolutely imperative that you:

  1. Do a complete tune-up. Plugs, points, wires, cap, rotor, and probably spark plug wires. (I recommend MSW stainless steel wires...my opinion)

  2. Check compression, adjust valves and ignition timing.

  3. Fill the tank with good gasoline of high octane, or add an octane booster. Gas required for 9.5 compression is about 97 octane. 8.5 is about 95 octane.

  4. Be sure the engine is warm before you start, but also be careful it does not overhear while you are making the adjustments. This can be dangerous, so have someone watch idle and temp while you are working or check them frequently yourself.

Familiarize yourself with the adjustments in the diagram. Remove the air cleaners. Screw both idle adjustment screws (# 1) all the way down and make note of the number of turns. (A turn for this article is 180 degrees. You may use 360 or any other: but be consistent, i.e. a point on the screw rotated 180 degrees is counted as a turn.) Bring both screws on both carbs up about four turns until you have an idle of 1000 rpm. Bring both #4 screws up and adjust until a 0.001 feeler gauge just slides beneath the screw head. Now, place a Unisyn gauge over both carbs and slowly turn #1, idle adjustment screws, until the airflow is equal in each carb. Speed the engine up to 2000 to2500 rpm and check the airflow with the Unisyn. If it is not equal you will have to loosen #5 and change the throttle relationship. Generally, this will not be necessary, but not always.

  1. Idle speed adjustment.

  2. Not adjustable, should be tight.

  3. Main mixture screw. Sets the jet level

  4. Adjust so that a 0.001 gauge slips through. Opens throttle when choke is pulled.

  5. Balances two carbs for throttle opening and air flow.

Shut off the engine. Remove the screws that secure the air chamber and remove it along with the pistons and spring assembly. Both sliding pistons should be full of oil up to 1/4 inch from the top. Look down at the brass tapered needle hole in the middle of the carburetor interior. Note that the inner brass piece is lower than its surrounding carrier. If you have HD type carbs, turn screw #3 on both carbs until the center brass tube is level with the outer brass housing. Count the number of turns and write it down on paper. This step is important so that you can always restore the carbs to where they were when you began and hence you will be no worse off for trying this procedure. I still have my original settings scribbled on a garage wall(seven turns up for the curious).


HD-8 carb adjustments

Now turn both screws down 10 turns. Whatever you do to one carburetor you must do the same to both!!! Replace the pistons and springs and secure the pistons on each carburetor. Start the engine. It should be grossly rich and idling a tad slow. Turn the mixture jet screwup one turn on each carburetor until the engine rpm rises and stop when the idle no longer increases. You may want to slow to 1/4 turns on each carburetor as you get close. When the idle no longer increases, stop. You are very close to the ideal mixture.


You may want to experiment with the knurled knob on the distributor and change timing a few degrees to fine tune the engine. The same applies to the carbs, but what you do to one, do to the other, and make small adjustments. If you coast down a long hill and the tailpipes backfire, screw the mixture up one turn. Adjust the # 1, idle adjustment, for the idle that you prefer, but turn them up or down equally. What you do to one carb, do to the other...keep them synchronized!


Your car should be running sweetly and smoothly. All the above also applies to earlier carbs but the jets have a large nut underneath to adjust the mixture. You can watch the jet come up level in the carb throat. Use the fiats of the nut for counting purposes.


It's normal to have to adjust the idle for summer and fall running. Air velocity carburetors are affected by air pressure. They run faster in cold dense air and slower in the summer heat. SU carbs are simple, and sweet. There are no small ports to get plugged up. It won't hurt to make sure your float chambers are clean and that the float level is adjusted per the manual.


This is the best way I know to adjust the carbs and they should never need major adjusting unless you rebuild the engine or the cylinder head. If your engine starts to run sour, the carburetors are the last thing to suspect. (Excluding bad gasoline. They are quite sensitive to poor gas. I recommend Super Shell or Amoco Premium...opinion again).


Ail this works for my car, and she purrs. Good luck to you with your SU carbs.


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