It is an idle air bleed. In restricts air flow, enrichening the mixture or
slowing the idle speed (forgot which).
Out is the reverse. It goes around the throttle butterfly, so the more you
open the throttle, the less this
circuit matters.
The inner screw is the fine adjustment, the outer nut is the coarse adjustment.
My Bentley manual contains
dire warnings of disaster if one messes with the outer (coarse) nut. It
supposedly is set at the factory,
requiring special tools and such. I ignored that of course, and see no reason
why it cannot be adjusted
successfully by a person who understands the difference between coarse and fine
adjustment.
So much for theory, as I was never able to get the blasted carburetor right
after I took it apart.!
I couldn't count the hours I spent arguing with that carburetor. It wasn't
even consistent in how it
would screw up and drift.
I finally gave up and put on a set of SU's. Best power modification I ever
made to that engine. I still don't have
all the bugs worked out of them either (needle size, heat shields, fuel
supply), but they sure beat the heck
out of the single ZS.
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