[6pack] Carburetors and Induction System

[Richard Lindsay]

Hello Friends,
   Here is a piece that I wrote a bit ago. Prompted on by those of you who
have responded, this and a following note, are repeated here. I hope you
find them informative.
   Although the assumption is that the engine is fed by SU carburetors, the
physics hold true for Z-S fed cars. Please ignore the minor irregularities.

Rick

Carburetors and Induction System

   Inside of the SU’s 'float bowls' are brass cylinders called 'floats'.
They float, thus the name, on the petrol in the 'bowls'. The float bowl is
plumbed internally to the main 'jet' or tube, sticking up through a
'bridge' in the 'choke'. The bridge forms a venturi, accelerating the air
passing over it.
   Physics tells us that faster moving air exhibits lower pressure causing
petrol, resting at ambient pressure, to flow up and out of the jet and into
the rarefied air. How much flows is determined by the metering 'needle'.
That closely fitting needle is tapered with the smaller diameter at the tip
and larger at the shank. The needle is attached to an 'air piston' or air
dam that raises and lowers as the 'throttle' demands air. More demand and
the air piston lifts, admitting more air. In doing so, the attached needle
lifts up the jet. As the diameter of the needle decreases, the annulus
between the needle and jet increases in area, allowing more petrol to flow
into the air stream. And Voila! The fuel is metered to the air, keeping the
'air:fuel ratio' of the 'charge' correct!
   Gravity makes the petrol in the jet seek the same level as that in the
float bowl. Therefore, the level in the bowl must be carefully established.
And that is the job of the ‘float bowl valves’. So the first step in tuning
is to install a new float bowl valve and adjust the actuating arm or float
feelers, so as to correctly set the petrol level in the bowl and jet. With
the SU, the correct setting is 11mm, as per the TR3b / H6 carburetor
manual. The height is measured from the lid flange up to the fingers with
the arm resting on the valve.
   The cork gaskets on the float bowl covers are available new but because
of low demand, they are typically old stock, and have shrunk a bit. They
just don't fit well. So instead of fighting new-old-stock, many tuners
reuse the old cork, helping it seal with a thin smear of vacuum grease -
the stuff used to seal glassware in wet chemistry labs. The other value in
reusing the old gaskets is that they are already compressed to the proper
thickness. While sealing well, the new gaskets may require the attachment
nut to be over tightened to attain the correct thickness.
   The SU carburetor, when in use, is operating in a dynamic state. The
fuel dynamics are actually very mild. Petrol flowing from the jet is
replaced via the float bowl valve. That's about it. The inertia of the fuel
causes the replenishment to lag the output. However, even at maximum flow,
the amount of petrol flowing is very small. Thus the mild fuel dynamics and
it is of no consequence.
   Air flow is another issue all together. Air (moving at low speed) has
very little inertia, owing to its low density. When the throttle is opened,
a great gust of air begins to flow. Without some method of moderation, the
mixture would lean-out terribly. The fuel flow simply couldn't keep up with
the air. But fortunately, the mighty SU moderates the air flow, via a
'damper' on the air piston.
   There is another piston down a bore in the center of the air piston.
That piston forms a damper because it contains a one-way valve. The little
bore is partially filled with a thin oil, for example, ATF. The upward
motion of the air piston is retarded by this damper and slows it down. This
is how the SU prevents the charge from leaning-out. The one-way valve in
the damper allows the air piston to fall quickly when the air flow reduces
as the throttle closes. So the second step in tuning is to add oil to the
dampers. The correct amount of oil is set when resistance is felt when
inserting the piston, just as the cap reaches the threads in the cover. The
cap is further tightened, if only snugly.
   The desired air:fuel ratio is set by raising or lowering the jet. This
is done by adjusting the jet height nut. Clockwise adjustment raises the
jet. Anticlockwise adjustment lowers it. Setting the initial jet height is
the next stage in tuning.
   The stoichiometrically correct air:fuel ratio for modern petrol is
14.7:1, by weight. That is, to perfectly burn the charge, 14.7 units of air
are required to burn 1 unit of petrol. That is also the ideal air:fuel
ratio for over-run, after 'lifting'. However, it is not the ratio that
produces the most power. Testing demonstrates that the peak in the power
curve occurs when the air:fuel ratio is about 12:1. Since this discussion
is about carburetors on sports cars, this is the desired air:fuel ratio.
      A discussion of the cold starting circuit is now appropriate, since
cold starting requires a change to air:fuel ratio. The word 'choke' is used
for two purposes. In the SU carburetor it means the central bore or air
path through the carburetor. On the dashboard, the control called the
‘Choke’ is an American-ism. The British term is a 'strangler'. However, SU
carburetors do not have air restricting stranglers. Rather, they induce
charge enrichment for starting by lowering the jet, thereby allowing more
fuel to flow past the smaller diameter needle position. But why do engines
need enrichment to start, especially when cold?

   The short answer is because petrol has to be in the gaseous state to
burn. The slightly longer answer requires a bit of preface. The engine bits
are (typically) made of metal and when cold, they absorb heat easily.
That's why they feel cold! So here's what happens at cold start.
   When the throttle opens, the 'charge' (air and fuel mixture, ideally at
about 12:1 ratio, by weight) rushes in. That's because the action of the
engine's pistons cause a reduced pressure beyond the throttle plate. When
the charge experiences the drop in pressure, the petrol evaporates. Think
lowered boiling point at high altitudes. All is as intended except when the
engine is cold.
   When the engine is cold, the petrol now vaporized by the reduced
pressure, condenses back to liquid on the cold metal walls of the manifold,
intake runners, head and valves. When it does so, this 'inlet track
wetting' leans the charge below the ignition point. So what is done to
remedy the problem? The charge can be enriched by strangling the air, as is
done in older American cars, or by lowering the jet in SU equipped cars.
And the later is precisely what the 'choke' control on the old British
car’s dash does.
   Once the engine is running, the problem doesn't immediately go away. The
engine and all of its inlet bits, have to 'warm up' before the 'wetting'
ends. To speed up this process, carburetors, inlet manifolds and often
heads, are made of aluminum. That metal conducts heat much better than
iron, allowing the inlet track to warm quicker. In fact, many early
Triumphs, MGs and Jaguars route coolant, straight from the water jackets
around the combustion chambers, to an isolated channel within the inlet
manifold. Again, the goal is to heat the inlet track, reducing warm up time.

   The first step in tuning multiple SU carburetors is to disconnect the
throttle linkage between the carburetors. This is done in order to tune
each carburetor separately before ‘synchronizing’ them. The carburetors are
then preset.
   The jet heights are pre-set to a starting point. This is done by turning
the adjusting nuts clockwise until the tips of the jets are precisely flush
with the bridges. The nuts are then turned anti-clockwise 6-8 hex nut
'flats'. The precise amount isn't critical at this stage, as long as both
carburetors are aligned by the same amount. The idle adjustment screws are
backed off until they just touch the linkage. The screws are then turned
clockwise for one full turn. These screw settings and the jet heights are
the initial settings. Start the engine. It should run with these initial
settings but a bit of enrichment with the choke control may be required
until the engine is warm.
   Once the engine is at operating temperature, use an air flow meter to
measure the air flow on each carburetor. Use the idle speed screws to
balance the airflow between the carburetors, and to attain a low idle
speed. It may not be possible to set a proper slow idle but try to attain
1000rpm or so.

   Many SUs have a small pin on the underside of the body, near the choke,
with which the tuner may lift the air piston a tiny bit. Other models do
not have this pin. With the later, a small screwdriver may be used to lift
the air piston. These pins or screwdrivers, are used to test the air:fuel
ratio of the charge.
   It doesn't matter which carburetor is tuned first. Make a choice. Lift
the tuning pin or manually lift the air piston a tiny bit and observe the
engine speed. An electronic tachometer attached at the engine bay is
helpful. When the air piston is lifted, the engine speed may change, and
then will probably stabilize. If the engine speed increases, the charge is
too rich, since adding extra air made for more efficient combustion. If the
engine speed decreases or tries to die, the charge is too lean. The
corrections are made with the jet height adjustment nut.
   If the tests indicated that the charge is too rich, raise the jet by
turning the adjusting nut one 'flat' clockwise. If too lean, adjust the
other direction. Record the results. In either case, test again with the
lifting pin or screwdriver. Continue testing and adjusting until lifting
the air piston causes no change in engine speed, then record the total
number of flats changed. Apply the same adjustment to the other carburetor
and re-test both. Small adjustments may still be needed owing to flow
dynamics and variations in carburetor manufacturing.

   Re-test and correct, as needed, the airflow balance and set the slow
idle speed to the value specified in the manual. Now, test everything
again, making corrections as required. Finally, tighten the linkage paying
particular attention to not disturb the settings. Test again.
   The final tuning step will have to wait until the engine is once again
cold. There is an adjustment screw on the carburetor and a cam on the
throttle for the choke circuit. When the choke is applied, the cam rotates
against the screw opening the throttle a tiny bit facilitating warm-up. Use
the screw to set it to the lowest cold idle speed that is consistent with
smooth running.
   Thus ends the discussion of the charge and carburetion.
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