Tuning SU Carburetors

mechanic girl fixing a car

SU Carburetors

[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

To tune SU carbs, first locate the following components:

  • Throttle linkage nuts. These are the things that connect the
    throttle linkage (the bar connected to your foot through whatever
    means your car uses, cables or rods) to the carburetors’ throttle


  • Throttle stop screws. These set the idle speed for each carb,
    and are located typically behind the dashpot, on the same side
    of the carb to which the throttle linkage connects.



  • Mixture adjusting nut. This is the lower of the two nuts at the
    very bottom of the carburetor. Later SU carburetors of the HIF
    type have integral float chambers, on which the mixture is
    adjusted by turning a screw. You’ll need to experiment (and I
    explain how) to see which way makes this richer and which way
    makes it leaner.



  • Lifting pins. These are little wobbly metal pins under the
    dashpot. When you push up on the pin, it raises the piston in
    the dashpot. Find these; they’re crucial if you don’t have a
    Colortune. If you don’t have or can’t find them, you can raise
    the piston with a flat-bladed screwdriver pushed down the throat
    of the carb and twisted to lift it.



  • The bridge. This is the part inside the carburetor, where the
    gas jet opens into the airstream. You’ll see a needle inside
    the jet, and the jet itself should be a few fractions of an inch
    down from the bridge itself. The jet is the brass tube that sits
    in the center of the bridge, with a tapered needle poking down
    into it.



  • The choke linkage nuts. Comparable to the throttle linkage nuts
    (and usually the same size), but on the linkage that goes between
    the choke cable and the mixture adjustment mechanism. They make
    sure that both carbs are enriched when you pull on the choke.Balancing The Air Flow1. 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


    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
    exhaust note and compare it to the following conditions:

    • If the exhaust note rises and stays high till you drop the piston,
      this carburetor is adjusted too rich. Turn the mixture nut one
      flat (one-sixth of a turn) up, moving the jet toward the bridge,
      then repeat Step 4.



  • If the exhaust note falls and the car sounds as though it is going
    to stall, this carburetor is adjusted too lean. Turn the mixture
    nut one flat (one-sixth of a turn) down, moving the jet away from
    the bridge, then repeat Step 4.



  • If the exhaust note rises briefly and then settles back down to
    something like the original RPM level, this carburetor is set
    correctly. When you have achieved this setting for both
    carburetors, continue with Step 5.


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
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

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
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
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. Do not strech the spring. When all is operating
properly, fill the damper with Marvel Mystery Oil for light damping, or use
motor oil for heavier damping. (I use MMO) If you get “flutter” on,
acceleration, you might try the heavier oil.

step 5-

Start the car and warm it up, then turn off/disconnect/otherwise
disable the choke mechenism. (Loosen the nuts on the clamps so that the
choke stuff isn’t doing anything) This will get set later.
(Later Zenith-Strombergs have a thermostatic choke, not a cable.)

step 6-

Check coarse throttle adjustments- make sure the throttle cable
pulls on both carbs equally, and returns completly when released. This
is adjusted by loosing the set screws on the throttle shaft and matching
the two sides. You can also adjust the cable length at this time, using
the cable set screw/retainer at the end of the cable. You can check the
float adjustments now by removing the piston & cover, and looking at the
fuel level in the needle seats. Both carbs should be about even, a little
below the top surface of the jet. If not, readjust one or both floats to
match the level.

step 7-

Syncronize the throttles- if you have a uni-syn, here’s your
chance to use it, (or other air flow guage), if not use a tube and listen
to the airflow. The Uni-Syn is much easier to use, and can result in
better balance. Alternatly adjust the idle screw on each carb, attempt
to set the idle as low as possible (~800-1000 RPM). Adjust until the
airflow is *close* to the same at each carb. The engine may now be running
rough, just keep the idle speed high enough to keep running. Give the
throttle a quick snap to make sure everything is settled, then check
sync again. Periodicly snap the throttle to make sure everything is seated.
Large differences in where you can adjust the two carbs may indicate
air/vacumn leaks, or other problems, such as a bad valve)

Magic Time- Relax, and shake your voodoo rattle…

step 8-

Adjust the mixture- this is done with the spring-loaded hex fitting
under the carb, where the fuel supply tube enters from the float. Turning the
fitting raises and lowers the needle seat. Pick a carb, and turn the fitting 3
flats (1/2 turn), first in one direction, then back 3, then 3 in the other
direction. Note where the engine runs better, idle speed should increase.
Turn to the best setting. Repeat this proceedure until you get the best
operation you can, (higest idle speed), keeping track of flats turned will
help you remember where you were. If you get lost, turn all of the way in,
then back out 12 flats and start again. Periodicaly snap the throttle and
push up on the fitting to make sure everything is seated.
Note: Type HIF carbs (With integrated float bowl) no longer have
the hex nut to adjust the mixture. Instead, there is a screw to twiddle,
on front of the front carb, and behind the rear. The screw is connected
to the needle seat through a temperature compensated gizmo, which is
said to make the carbs more stable. Adjustment can be done in much the
same way, by counting turns/flats of your screwdriver. There is less
adjustment range than with the the basic models.
When you think you’re close, stop, uncramp your fingers, breath deep,
and do the same to the other carb. Then retune the first carb, and then the
second again. This serves to match the mixture of the 2 carbs, and prepare
you for the beer sitting over there in the sun. (why do you think the British
drink warm beer?)

step 9-

repeat step 7, setting the idle speed as low as possible, and
re-syncing the idles. Now go back and readjust the mixtures. After a couple
of iterations, the engine should be running smoothly (controlled by mixture)
and at a low idle. Repeat as necessary. Set the final idle to 800-1000 RPM,
depending on the condition of the rest of the engine.

This is a standard mixture test, performed AT IDLE:
Under operation, (air filter off) lift the carb piston by 1/16″ with
the lifting pin or a screwdriver, which leans the mix a tad.
-RPM’s rise and stay up, that carb is rich.
-RPM’s rise briefly, then drop, mix is about right.
-RPM’s fall, engine gets rougher- mix is lean.

[where is step 10–Ed]

step 11-

Adjusting the choke- I won’t get into the temperature compensation
in the type HIF, or the Thermostatic choke in the later strombergs.
Check the manual for more info.
The choke is supposed to do two things; the first half of travel moves a
cam on each carb which opens the throttle, for warm up. The second half
pulls down on the needle seat to enrichen the mixture, for starting.
Start with the choke in the off position (knob in). Adjust the
so that the cam only starts moving the throttle after you start pulling
out on the cable (adjust with shafts and adjusting screws). Try to get both
carbs adjusted the same, so that both screws begin to hit the cam at the
same time. This is not real critical, but you can use your Uni-Syn to
match air-flow on both sides, with the choke partly engaged.
After the cable is about halfway out, it should start engaging the
lever which pulls down on the needle seats. Adjust the linkages so both
carbs are acted on equally. You can do this by adjusting for even running
of the engine. Of course, for a warm engine, the richness of this mixture
will cause some roughness. Make sure the needle seats return freely
when you release the choke.

step 12-

Drink that warm beer (only one, no DWI now…) it will taste great
at this point!, go wash up, and go for a ride.


These procedures assume that your engine/carbs are in reasonable operating
condition. If something is malfunctioning/leaking, etc, this should still
help, but the results may vary. For instance, if you have leaky carbs, worn
needles, engine modifications, etc, you may find things work better if
you tune for optimum performance at open throttle rather than idle.

The first time through carb adjustments can be confusing, once you’ve done
it, all of the stuff in the manuals makes sense. Go back and read them again-
As always, I recommend Bentleys, which is a repro of the original factory
manuals, and then Haynes, and throw out the Chiltons. (orginal factory manuals
are to be read in a clean enviroment, repros are for smearing grease all over,
except, if that’s all you got, use it!) Haynes has an excellent manual just
for SU carbs, it covers operation, theory, rebuild of all models, and has
needle charts for hundreds of car/engine/carb setups. They also have a
manual for Zenith-Strombergs, which, while similar, are a whole ‘nother beast.
-Saftey Fast!

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One thought on “Tuning SU Carburetors

  1. Puddinhead

    Regarding the fast idle screws; other than matching multi su card throttles closed properly, what else does the fast idle do ?

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