A flat is 1/6 of a turn of the nut. It refers to the flat sides
on the nut -- the parts that the wrench grips. (A little simple
math, as so amply demonstrated when my esteemed colleague from the
noble and storied township of Ben Lomond got the multiplication
factor for dollars to pounds sterling reversed, would point out
that when I say "twelve flats, or two full turns of the nut," I
give the key in my posting. Whoops, I've been listening to the
Senate for too long...)
The other keys to this are to scribe a mark in one of the flats
that is facing outward, so that you can see it, when the adjusting
nuts are all the way at the top of their travel. Then to start
the tuning procedure, turn the nuts until this marked flat has
come around twice. That's twelve flats, or the place to start
your carb tuning.
After that, when you're seeking the correct mixture (either by
ear or by Colortune), turn the mixture adjusting nut only one
flat -- one-sixth of a circle, 60 degrees -- between tests. The
flats give you a ready visual cue so that you know that you are
turning each carb a repeatable increment in either direction. You
can also get an idea for whether one carb (or one of the cylinders
being fed by the carb, in extreme cases) is defective by keeping
track of how many flats you deviate from the base. If one carb
works best at 13 flats down from the bridge while the other one
requires 16 or 18 flats, you've got bad air leaks on the latter
carb, bad enough that the engine probably won't run well no matter
what you do. Time for the oversize throttle shafts.
I don't know how much each flat drops or raises the jet, but it's
a truly minuscule amount.
--Scott
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