I really don't think that you can pin the rose on the tach. For most B18s/B20s,
a bit above the slowest speed at which it will continuously run is a fine speed
to set things at, so if it slows a bit, it won't die. Actual idle speed is not
that critical as long as it isn't way too high.

If you have KD needles that are in good shape I would use those. ZH needles are
usually used on the earlier models and "A" models with lower-speed cam and single
carb.

If there is a problem with ignition, it would have to be either bad dwell, retarded
ignition or variable timing (worn distributor) with heaviest emphasis on retarded
ignition. It might be worthwhile to make sure your timing marks are right by
looking/feeling through #1 plug hole and getting the piston to the "real" top
and then seeing if the timing mark is really at zero.

Rev. Dr. Shannon sets his by ear and does well. I have done that and then checked
and found that I was within a degree or two of the spec, so it is not a bad way
to go, especially if you doubt what you have.
--
George Downs, Bartlesville, Heart of the USA!