Despite its simplicity, the SU really is pretty good at mixing fuel and
air. There are better units, admittedly, but they cost a lot more, have
loads more pieces, and take at least as much maintenance. The needle IS
the important thing. If the air passages are clear so that the piston can
rise and fall as intended, the needle will do its job. They do wear, over
time. Treat yourself to a new set. What is remarkable about the SU is
how well it enriches the mixture during acceleration. The piston would
rise quickly, in response to the increased air flow through the carb,
were it not for the oil-filled damper on the top. Experimentation with
different weights (sae 10, 20, 30) etc plus new needles may give you the
results you seek. IF the throttle shaft is loose (shake it) air will
bleed past the shaft and make adjustment difficult and mixture strength
inconsistent. Replacement of the shaft or bushing installation is a job
for a shop, but you can test the effect by squirting heavy oil on the
shaft where it enters the carb. Lubrication here won't hurt anything, yet
noone ever does it. A test for leakage is to idle the engine and play a
stream of propane from an unlighed Bernztorch on the shaft. If the engine
speed increases, the shaft is passing air. Consider the number of little
moving parts that wear in any other carb and the age of the SUs that are
workign on your MG and they don't compare too badly.
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