I just read the following in a Suburban manual:
BURNER ADJUSTMENT
To adjust primary air to the main burner, the
small sheet metal cover found just below and to
the right of the lighter opening must be removed .
Behind the cover is a slotted screwhead . With a
screwdriver, turn screwhead counterclockwise
for less primary air and clockwise for more primary
air . A symptom of too much primary air will
be a howling or screeching noise when the burner
is on (reduce air to correct) . A symptom of too
little primary air will be sooting on the exterior
vent and a distinct yellow and floating flame
(increase air to correct) . A hard blue flame is
the sign of correct adjustment . NOTE : If a sooting
condition cannot be corrected by the air adjustment
on the burner, discontinue use of furnace
until problem can be corrected by a service
agency .