The differences between needles are pretty small, and the basic mixture
is adjusted by the jet postion, which makes a LOT more difference than
which needle you use. What is accomplished by changing needles, then?
you ask. The needle profile changes the amount of mixture variation when
the carb piston is lifted, so if it gets richer too fast, then you need
a more barrel-shaped needle, and if it doesn't get richer fast enough, you
need a more trumpet-shaped needle. Unless you have a perfectly-tuned engine
and a pretty accurate way to measure how rich your mixture is, you would
probably not be able to tell the difference between needles.

I use the colortune device and still have a hard time telling any difference,
even using two of them at a time to compare front and back carbs. Other
problems, like jet adjustment, float level, clearance around the vacuum piston
and even vibration, seem to make a bigger difference than choice of needles.
BTW I am using KD needles. Usually I get good mileage, sometimes even with
a leaking carb, and other times I get bad mileage, but I don't think you can
pin the rose on your choice of needles.

If you have vacuum advance and set your timing to 10° btdc with the
vacuum disconnected, and then reconnect the vacuum, you have effectively
set it to about 20°. I set it to 20 on both cars, on the 164 without
disconnecting the vacuum, and on the 122, it doesn't have vacuum advance.
The big thing the vacuum advance does is to keep the engine from kicking
back against the starter. My 122 sometimes kicks back slightly but not
enough to keep it from starting. Once it starts it runs great, usually
starting in about 1-3 seconds unless the temp is below about 20°F, in which
case it might take 5-8 seconds. Thinner oil would probably help.
I do set the timing at idling speed, probably around 750-800 rpm.
--
George Downs Bartlesville, Oklahoma