British
Sportscar Centre
Setting
up your carbs.
You
may wish to print out this page for future reference.
1. Basic
Adjustment Strategy
The
trick to tuning Skinners Union (SU) or Zenith Stromberg (ZS) 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.
SU carbs were used on the early MG's, while ZS carbs were used on the
later ones. The design of each is quite similar; thus adjustment is
performed in the same manner for either type. If you would like to read
more detailed information about how your SU or ZS carbs work, there
are excellent Haynes manuals for each of these carburetors.
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 $35 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, one helpful device is called the Gunson ColourTune.
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 $75, 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, the standard directions are included here
for determining correct mixture (step 4 of the Adjusting Mixture
procedure).
-
For
some ZS carbs, you will need a special mixture adjusting tool. This
is a special two-part wrench that is inserted through the top of
the dash pot of the carburetor to adjust the metering needle while
preventing the air valve from rotating within the carburetor.
Terminology
To
tune SU or ZS 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, near the connection of the throttle
linkage.
- Mixture
adjusting nut. On early SU carbs, 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. Some ZS carbs do not have adjustable mixture.
ZS carbs that do allow mixture adjustment either you'll need to
experiment (and I explain how) to see which way makes this richer
and which way makes it leaner.
- Lifting
pins. (SU carbs only) 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 them, can't find them, or have a ZS carb, you
can raise the piston with a 1/16" drill bit inserted in the
throat of the carb to lift the piston.
-
The
bridge. This is the brass part in the bottom centre of 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. Your car
may have no choke linkage, but a split choke cable which connects
to each carb, in which you can forget about these...
2. Before
You Even Touch the Carbs!
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. All of these things can affect the setting of the carbs,
which should be adjusted last, if at all! After being properly set,
the carbs should rarely need further adjustment. If you've got a timing
light and a dwell meter, you can verify the ignition components independent
of the way the car is running. When it's warm, shut the motor off and
remove the air filters.
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!
3. Balancing
The Air Flow
If
your car has multiple carburetors, the air flow needs to be balanced
amongst all carbs before the mixture is adjusted. If you have only one
carb on your car, you can proceed directly to mixture adjustment!
1.
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.
2.
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.
3.
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.
4.
Place the Unisyn on each carburetor in turn to check its flow, adjusting
the throttle stop screws until all carburetors register the same position
on the graduated tube ofthe Unisyn. (The float will probably move either
up or down in the tube, which is why you want to center it in Step 3.)
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.
4.
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.
These
instructions are for SUs with separate float chambers or ZS carbs, but
the same practice will work for later model SU with integral float chambers.
You will need to check in your shop manual to see whether you turn the
mixture screw to the right (in) which is leaner or the left (out) to
make it richer, some carbs are different.
1.
Shut the car off and loosen the choke linkage nuts, if you have any
to loosen!
2.
Adjust the mixture nuts (screws) fully lean, or flush to the bridge.
Check your shop manual to make sure you are adjusting them the right
way!
3.
Now enrichen each carb(s) by equal amounts -- two full turns of the
adjustment nuts (screws) will be a good starting point. 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 drill bit 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 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 all carburetors, continue with
Step 5.
5.
Tighten the choke linkage nuts, if applicable, 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 little bit of a test drive!
5.
Special Notes
SU
and ZS 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 ColourTune, 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. SU and
ZS carbs 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. For light damping, 3 in 1
(sae 20) Oil is excellent, engine oil (sae 20w50) can be used for heavier
damping.
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 carbs 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 and ZS 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. The standard failure for Grose-Jets is to flood the carburetor.
Cheers,
Trevor.
©2008
Photogenix all rights reserved.
©1999-2008 British Sportscar Centre.
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