I know that Roger Garnett has an acclaimed write-up on tuning SU carbs,
but here's the distillation of my experience on the subject. I just
wrote it up for a couple of other lists, and it certainly applies here.
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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 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 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 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!
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.
--Scott "I like SU carbs -- they're expert-friendly" Fisher
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