Ray Livingston’s SU Carb Adjustment

Here is the unedited text of Ray Livingston’s “famous” thread on the Jag-Lover’s forum, regarding the method for adjusting SU Carburetors:

Just go by how it drives. If it’s rich, you’ll get poor mileage, an erratic idle, and a ”burbly” or ”splashy” exhaust note, plus, usually, popping from the exhaust on over-run, or coasting down from high RPM in 2nd gear. If it’s lean, you’ll feel it surging when cruising at speed, it might run hot, be hard to start, need the choke on for along time when cold, and stumble and perhaps even backfire through the carbs when you try to accelerate.

The easiest way to tune it is to get a large analog shop tach, and tune for highest idle speed. Set the idle as low as possible (300-500 RPM), and tune carefully for highest idle speed. That will usually get you very close. Start by setting all the jets flush with the bridge, then crank them down 2.5 turns. Turn all equally, while watching the idle speed. Start by turning all three a full turn richer, then leaner, and seeing which gives better idle. Then go to 1/2 turn and repeat, then 1/4 turn. Once you get the best idle you can moving all three, only then start tuning each individually, starting with 1/2 turn, then 1/4 turn. Many people have gotten better results using either UB or UE needles, as UMs are not really ideal for unleaded gas, and tend to be much too rich at idle when tuned for proper higher-end mixture

The length of the two pushrods – the vertical one coming off the pedal arm, and the one running across the firewall – have a large impact on the throttle response. Look at the crank arm where those two connect in front of the driver. You want the ball joint for the horizontal link to be pointing towards the passenger side of the car when the throttle is closed. That way, it will take a relatively long pedal motion to move the throttles significantly. When you approach WOT, the ball joint should be above the crank arm pivot, if not over to the drivers side, giving a larger throttle movement per pedal movement. You can adjust the lengths of the two links to get it where it needs to be..

1) Set the idle speed AS LOW AS POSSIBLE, usually about 450-500 RPM should be possible. The lower you get it, the easier it will be to set mixture correctly.

2) Using a Uni-Syn, make sure the airflow on all three carbs is balanced

3) Using a good, sensitive electronic tachometer, adjust each mixture screw, looking for HIGHEST idle speed. Do the adjustment first in 1 turn increments, to narrow down the range, then in ½ turn increments.

4) After doing step 3, start over again from step 1. Keep iterating until it is no longer necessary to make any adjustments in steps 1-2. At this point, make the mixture adjustment in ¼ turn increments. You may find at some point that 1/4-1/2 turn increments don’t make a noticeable difference. In this case, find out how far you have to turn the adjustment in BOTH directions to make the idle speed start to drop, and set the adjustment to the MIDDLE of this range.

5) Finally, reset the idle speed to whatever speed gives the smoothest idle, usually around 650-750 RPM.

While making the adjustments, change the mixture, and watch the idle speed for at least 5-10 seconds. It will take a bit for the change to have maximum effect. Also, blip the throttle occasionally, taking the engine to at least 2000-2500 RPM for a few seconds to clean it out, then wait for the idle to stabilize before continuing.

At this point, you should be *very* close to the optimum mixture. Drive the car, and see how it runs. I find mine usually ends up just very slightly lean using this method, but yours may be different. If it is, you may find when you coast down a steep downhill in a low gear (at 2500RPM or so), you’ll get some ”popping” from the exhaust.

Now go back to your garage, and listen carefully to the exhaust. You may hear random light ”popping” (if it’s regular, you have an ignition problem). Now comes the real fine-tuning. I find these things are *really* sensitive to idle mixture, and this popping is your best indicator. First, see which pipe is popping. One pipe popping indicates either the front or rear carb is off (I forget which is which, just follow the exhaust pipes up, and see which is connected to the front cylinders. If the popping is occurring simultaneously in BOTH pipes, then the center carb is off. Adjust the mixture of the offending carb in 1/4 turn increments, and see what effect it has. When the popping goes away, you’re all done.

FWIW – I’ve *never* been able to get a good result using the lift the piston method. For one thing, it’s *really* hard to lift the piston only 1/32”, and any more than that will *always* drop the speed. I do know it ONLY works with a VERY low idle speed (like 400 RPM), and even then it’s very imprecise. I’ve used the method above for several years on several cars, and find I can reliably get the mixture ”just right” in about 15 minutes.