[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 ]
Contents
Tuning your SU carburettors by Scott Fisher
&
Tuning
Your S.U. Carbs by Roger Garnett
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:
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 levers.
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 air stream. 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 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:
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 doesn't'
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!
Notes
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.
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, recommended
reading. The basic syncing process also applies to Zenith-Stromberg's,
but the adjustment mechanisms
are different. Here is a layman's 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 across 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 carburetor 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 stretch
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 mechanism. (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
completely 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-
Synchronize the throttles- if you have a uni-syn, here's
your chance to use it, (or other air flow gauge), if
not use a tube and listen to the airflow. The Uni-Syn
is much easier to use, and can result in better
balance. Alternately 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.
Periodically 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 procedure until you
get the best operation you can, (highest 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. Periodically
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. If: -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.
Notes: 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. (original factory
manuals are to be read in a clean environment, 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
another beast. -Safety Fast!
Carburettor Dampening
from the eMOG
Pub
By "under-damped" do you mean the piston isn't moving
or is moving more than it should? The car
will run very badly if the dampers are dry. The
oil in the dampers smoothes out the movement of the pistons. Usually
only one goes dry and now half the engine is right and the other
isn't. As to being lean enough to raise the temp, I'm not sure. An
undamped piston should move higher and quicker in
response to load making it richer (pulling the needle
out further) but the velocity of air across the jet will drop (remember
this velocity is approx. constant) causing less pressure depression
and less fuel and a lean situation. I fill the dampers to about .75
inch below the top using ATF. It seems to offer the right weight
and is a high detergent oil that seems to keep things cleaner.
How is you temp now that you have oiled the dampers?
Tim (out exercising my dampers)
Factory recommendations may be motor oil, or special damper oil.
Solex Carb or SU Damper oil, available from some mail-order places.
For light damping (faster response):
Marvel Mystery Oil (very good
for this application)
Automatic Trans Fluid
3 in 1 oil
For moderate damping:
20 or 30 w Engine Oil
20-50 wt Engine Oil