ll the way in, remembering to
never use choker longer than necessary, as when not needed it has a
tendency to foul up engine and ruin the lubricating oil in the crank
case.

Next, set air screw for good idle by either turning to the right a
little or backing out to the left as the needs of the engine require,
remembering that first of all, the needle must be set as described. With
the needle so set and the engine warmed up, the adjustment of the air
screw for proper idling is easily accomplished by using a little care.
If the air screw is turned in too tight, the motor will roll. If the air
screw is not tight enough, the motor will hesitate and perhaps stop
entirely. To make a nice clean adjustment for idle, first having set
needle as described, turn air screw in quarter of a turn at a time until
engine, does roll; then turn back to the left until engine hesitates,
indicating that mixture has too much air and is too lean; next turn air
screw in to the right three or four notches at a time until engine runs
smoothly. This accomplished (and it is very easy to do by proceeding as
directed above) the proper adjustment for the entire range of the engine
will have been attained, thus insuring the best economy and power.

MODEL "S" MARVEL CARBURETER

[Illustration: STANDARD EQUIPMENT 1923-24 Series 691 Nash Sixes]

If the engine idles too fast with throttle closed, the latter may be
adjusted by means of the throttle lever adjusting screw.

RICH MIXTURE

An over-rich mixture will cause the engine speed to fluctuate through
more or less regular periods from high to low speeds; the engine will
seem to be mis-firing and there will be noticeable a strong odor, as
well as, usually, a heavy black smoke from the exhaust.

LEAN MIXTURE

The best adjustment is obtained with the fuel and air valves set as
described. It must be remembered that too lean a mixture as well as an
over rich mixture causes over-heating and loss of power and is not as
economical as an adjustment which provides just the proper proportion of
gasoline and air.

CAUTION

It must be remembered that the low speed needle has been carefully
calibrated to notch in disc handle and guide post above it, at the
factory and that in making an adjustment that the needle must be so set
and the rest of the adjusting done with the air screw as described,
never varying from described needle setting unless in extreme cold
weather, when a little more gas may be