I have tried both methods and have gone back to the "MGB" method. The
reason is that to disable the carb, you have to close the butterfly
completely then lift the dashpot. If the butterfly stays open or your
throttle shafts leak, you get too much air through the dead carb and the
other carb won't supply mixture through the balance tube and the engine
dies.
In fact, many will argue that no appreciable fuel goes through the balance
tube since it is mounted high on the manifold and, as such, will not carry
the river of liquid fuel that runs down the floor of the manifold. I have
never had any luck with disabling one carb so this may support this theory
or it may just show that I drive cars with worn throttle shafts.
The first step to successfully tuning SU carbs is recognizing what happens
to the mixture when the dashpot is lifted. At any rpm, the dashpot is
lifted by the difference in pressure between the air flowing under the
dashpot/over the jet bridge and the atmosphere. It is set up to keep a
"constant depression" or constant difference in pressure between these two
areas.
At the same time the float bowl, which is open to atmosphere, is
maintaining a fuel level just below the jet bridge surface. When the
engine is running, the difference in pressure between the float bowl and
the jet bridge lifts fuel into the carburetor and engine. So you have fuel
being forced into the engine under a constant pressure- the difference
between the pressure at the jet bridge and the atmosphere. With no other
control system, then fuel would constantly flow at the same rate into the
engine. This wouldn't work so SU added a needle and jet assembly that
constricts the fuel flow based on piston height. At slow engine speeds the
piston barely rises to develop the proper pressure difference so the
tapered needle in the jet restricts flow. Raise the speed and the needle
lets more fuel through by opening the jet while the pressure difference
remains stable.
When you lift the dashpot, you do two things. First, you open the venturi
a bit and reduce the pressure difference between the float bowl and the jet
bridge. Second, you lift the needle and thus reduce the resistance to fuel
flow. The first effect, lowering the pressure difference, seems to be the
most important so lifting the dashpot leans the mixture. However, it does
not take much of a lift before there is no longer enough pressure
difference to raise fuel above the jet bridge and the carb becomes
"disabled" and no fuel flows.
I have heard it said that when you lift the dashpot 1/32 of an inch, you
are testing the carb you are working on. If you lift it more, you are
testing the other carb since you have disabled the carb you are
manipulating. When setting carbs, I lift about 1/32 of an inch (other carb
not disabled) and tune until rpm's just rise with that movement. I then
confirm/ fine tune the adjustment by reading the plugs since, in my
opinion, idle mixture is not as important as cruise/full power mixture.
An interesting note of history on this. Apparently the standard procedure
for adjusting SU's was originally the lift one carb while ignoring the
other. Then in the mid fifties it changed to adjust one carb while
disabling the other(s). This change seems to coincide with some Jaguar
models being equipped with three carbs. Apparently the "disable" method
worked better when more than two carbs were involved. Interesting that the
MGB reverted to the older practice.
Regards,
Bill Eastman
|