Don---Biased needles are spring-loaded to one side. They are encased in
a sleeve that has a locating slot at one side, which keeps the needle
from turning when the needle height adjusting screw is turned.
The straight needles should be centered so they do not touch the side of
the jet at any time. Because the shank part of these needles are smaller
than the housing that is used for biased needles, a bushing of some sort
must be made to make up the difference. A similar type of set screw
keeps it in place. Some vertical movement is still possible to fine tune
the idle station that you want this needle to rest at.
The taper of both styles of needles rely on being the right shape for
the engine demands. We can only adjust the needle up or down to suit the
idle. From there on, the taper takes over, as you already knew.
I believe what Kas did in substituting SU needles would work with any
three styles of ZS carburetor.
Gotta go. Santa's coming!
Dick
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