...if that were the case (air leaking past the threads),
on a needle valve, then every needle valve ever made
would be defective. I don't think so.

The side inlet is the inlet side for the fuel, and the end
is the carb side. Real simple.

And " IF " there was an air leak through the threads of a
needle valve ( that would remain constant ) probably one
click of richness would compensate for it.

Dave.