What causes that
problem is the end of the spray bar is bell mouthed. The tapered
idle needle is in too far so metering is not proper.
You can adjust
it by coining the end closed. You can use a flat end punch
tap slightly tap the end of the spray bar while the opposite
side is flat against a steel block.
You want to close
the hole until the tip if the idle needle jut fits in the
end of the spray bar about .030" . Then try running the
engine. If its too rich right above idle increase the size
of the which can be done by pushing the idle needle into the
spray bar slightly to bell mouth it again but not to the degree
prior to coining it down.