SU Carburettors

[Did you know that S.U stands for Skinners Union. The Skinners are the brothers who turned a family leather business into making very successful carburettors. The S.U carburettors have figured in a large number of vehicles from the beginning of this century to the early 1980's. The earliest S.U's had leather bellows instead of the piston to slow the rise of the air-flow....... Salman SHAMI ]


1. Tuning your SU carburettors by Scott Fisher,

2. Tuning Your S.U. Carbs by Roger Garnett

1. Tuning your SU carburettors by Scott Fisher,

I've been meaning to write this up for some time, ever since I did the SU Performance Tuning 101 a few months ago. This one is more like Basic SU Adjustment for Happy Driving.

The trick to tuning SU carbs is to understand that there are two things you need to get right: the air flow, and the fuel mixture. While they are interconnected, they are also independent, and need to be measured and adjusted independently.

Special Tools

You will probably need to arrange to buy or borrow a Unisyn flow meter. The Unisyn is the usual gauge for getting the air flow balanced between the two carbs. This costs about $20 and is simple to use. It consists of an adjustable opening (same size circumference, but with a disc on a threaded rod that you can screw tighter or looser) that you use to set the level of a little float that rises or falls in a glass tube at the side of the gauge.

For the fuel mixture, I have become sold on a device called the Gunson ColorTune (maybe ColourTune, as it's a British co.). This is a spark plug with a crystal pressure- and heat-resistant window in it that lets you see into the combustion chamber while the motor is running. The color of the flame indicates the mixture richness. It costs about $40, and while it's not absolutely essential, it makes life so much easier that it's worth the cost.

If you don't have a Gunson, I've included the standard directions here for determining correct mixture (step 4 of the Adjusting Mixture procedure).

To tune SU carbs, first locate the following components:

Balancing The Air Flow

1. Start with the engine warmed up to operating temperature and perform your standard ignition tune-up (points gap, timing, spark plug gap, new condenser, etc.) first. If you've got a timing light and a dwell meter, you can verify all that stuff independent of the way the car is running. When it's warm, shut the motor off and remove the air filters.

2. Begin by balancing the air flow. To do this, first loosen the throttle linkage nuts. Leave them connected, just loosen them half a turn or so.

3. Back out the throttle stop screws till you can see that they are just touching the throttle stop. Then open each carburetor (that is, lower the throttle stop screw) 1-1/2 turns of the throttle stop screw and start the engine. It will probably idle at about 2000 RPM; don't worry.

4. Put the Unisyn over either carb and adjust the orifice in the Unisyn till the little float at the side rests at the middle of its graduated tube. (Pre-diagnostics: if the idle drops and the car wants to die when you slap on the Unisyn, the carb is too rich; if the idle soars upwards, it's too lean.) Hold the Unisyn over the carb for only long enough to see the level of the float, then remove it.

5. Place the Unisyn on each carburetor in turn to check its flow, adjusting the throttle stop screws until both carburetors register the same position on the graduated tube of the Unisyn. (The float will probably move either up or down in the tube, which is why you want to center it in Step 4.) When both carburetors flow the same amount of air, tighten the throttle linkage nuts, adjusting for the amount of free-play between the linkage and the throttle stops that your manual calls for (probably about 0.006"). Your goal should be to achieve the lowest possible idle with both carbs balanced and the engine running smoothly. (Note that the idle speed will very probably rise as you get the mixture correct.)

If you've taken more than five minutes to do this, rev the engine to over 2500 RPM (assuming the idle isn't already that high) for thirty seconds or so to clear the spark plugs. Then adjust the mixture.

Adjusting The Mixture:

Note: in the following procedure, one "flat" is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut.

I'm going to give instructions for SUs with the separate float chambers. If you have the HIF integral-float carbs, you'll have to look in a manual to see whether you turn the mixture screw to the right or the left to make it richer or leaner; I've done that once but I can't remember. Alternatively, you can -- with the motor shut off -- peer down the throat of the carb and turn the mixture screw while watching the top of the jet. Remember that moving the top of the jet up will lean out that carb, while moving the top of the jet down will richen it.

1. Shut the car off and loosen the choke linkage nuts.

2. Adjust the mixture nuts (screws) fully lean.

For separate float-chamber cars, this means raising the mixture nut all the way up against the bottom of the carb (or rather, against the spring). For HIF carbs, you can try turning the screw while looking down the throat to see which way the jet is moving. In either case, the idea is to zero out the jet: raise it all the way up in the bridge.

3. Now drop the jet an equal amount -- two full turns for HS-type carbs, two full turns (I believe) for HIFs. Then start the car.

Note: In the following step, you might want to consider adjusting the carburetors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process.

4. Raise the lifting pin (or use a screwdriver if you don't have the pins) so that the piston rises no more than 1/16". Listen to the engine's exhaust note and compare it to the following conditions:

5. Tighten the choke linkage nuts so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob.

6. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesnt' even touch the throttle stop, then use the other to get the idle speed right. When that's done, you can screw the other stop screw down till it just touches the stop on that carb and you're set.

7. Replace the air filters and go for a test drive!


SU carburetors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine's condition is and make fine adjustments to the mixture nuts accordingly.

If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburetor for 1 & 2, the rear for 3 & 4). The ColorTune will let you see the color of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power.

You can also modify your car's throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SUs are set up so that a thicker oil will resist the piston's attempt to rise in the dashpot for just long enough that the engine's increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily.

If you modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads.

If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine's temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old SUs is wear in the throttle shaft area. To test for this, spray some carburetor cleaner on the outside of the throttle shaft; carburetor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburetor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well.

Other problems that SU carbs experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn't stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down tight. Also, don't swap the pistons between dashpots; they're matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling.

Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure.

Some older SU models also have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass rod. These carburetors were replaced in the mid-1960s with carburetors that had fixed, plastic floats which are basically trouble-free unless abused. The stop at the back of the floats can break if they are installed badly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are fairly inexpensive and aren't a bad idea if you're doing a rebuild.

Grose-Jets are very popular with some people and a big pain for others. It appears -- and this is just conjecture -- that Grose-Jets work best in cars with adjustable floats, as they are longer than the stock SU float valves. The standard failure for Grose-Jets is to flood the carburetor. I have never had problems with the stock SU float valves or floats.

2. Tuning Your S.U. Carbs by Roger Garnett

Well, it's not really that hard to set up SU's, just different. Of course it always gets more interesting when you have more than one... There is a very good Haynes SU carb manual available, reccommended reading. The basic syncing process also applies to Zenith-Stromberg's, but the adjustment mechanisms are different. Here is a laymans guide to adjusting SU's (long):

step 1-

Tune up the rest of the engine- REALLY! clean or replace, and set the points, set the timing, plugs, valve lash, and remove the air filters. (have new ones ready) All of these things can affect the setting of the carbs, which should be done LAST, (if at all). The carbs rarely need to be adjusted, once set. Also replace/install the gas filter. Of course, it helps if the carbs are in good mechanical condition as well. But you can consider a rebuild once you have gotten things working first!

step 2-

clean the carbs! use gum-out or similar stuff, clean all external linkages, shafts, and stuff.

step 3-

Remove the float bowl covers, clean the float bowls, remove old sediment, and check/adjust the float setting. (turn the cover upside down, and get a *1/8" in drill bit, set the drill bit accross the cover, the float tab should just touch the bit.) Make sure the needle is moving and seating properly. This is just like *most* floats. Replace the cover.

* This is for HS4 SU's- (1/8-3/16") if you are dealing with 1", H's, HS2's HS6's, HIF's, etc.- check the spec for your carb.

Note: You can check for matching float settings, after setting the mixture, by removing the pistons, and peering down at the jets. The fuel level should be about the same on both carbs, a little below the top surface of the jet. (After car has been run only)

step 3b- Go get a pint of ale, or something close, and set it nearby.

step 4-

Remove the piston covers. CAREFULLY remove the piston, DO NOT BEND THE NEEDLE. Set the piston down on a clean wadded rag to prevent rolling. Clean the inside of the carb. Check operation of the throttle. Check the throttle shaft slop- this is the most common place for wear on an SU, and is often where air/vacumn leaks occur. The bushings and shafts can be replaced, but it requires some machining. A small amount of leakage can be tolerated, the car just won't idle as evenly. Clean the piston. Stare in awe at the odd carborator design, simple and effective, (constant velocity). Dump the old oil out of the damper if you haven't already spilled it. clean. Reassemble, check piston movement, raise it, then let go, it should fall freely. If not, check assembly again, make sure the piston isn't binding against the carb body, it should ride only on the damper shaft