TM 5-4240-501-14&P The  air  pressure  above  the  fuel  in  the  bowl  pushes  the fuel   down   in   the   bowl   and   up   in   the   nozzle   to   the discharge holes.  At the same time the air rushes into the carburetor   air   horn   and   through   the   venturi   where   its velocity is greatly increased. The nozzle extending through this air stream acts as an air foil, creating a still lower pressure area  on  the  upper side.    This  allows  the  fuel  to  stream  out  of  the  nozzle through   the   discharge   holes   into   the   venturi   where   it mixes  with  the  air  and  becomes  a  combustible  mixture ready for firing in the cylinder. A   small   amount   of   air   is   allowed   to   enter   the   nozzle through   the   bleeder.      This   air   compensates   for   the difference   in   engine   speed   and   prevents   too   rich   a mixture at high speed. The   story   of   carburetion   could   end   right   here   if   the engine  were  to  run  at  only  one  speed  and  under  ideal conditions. However, since smooth economical operation  is  desired  at  varying  speeds,  some  additions must be made to the carburetor. The ideal combustion mixture  is  about  14  or  15  pounds of    air,    in    weight,    to    one    (1)    pound    of    gasoline. Remember  that  an  engine  operating  under  heavy  load requires a richer mixture than under light load.  In order to  regulate   the   mixture,   we   place   in   the   carburetor   a threaded    needle    valve    with    a    tapered    point    which projects into the end of the nozzle.  See Fig.  16. To  adjust  the  carburetor  for  maximum  power,  run  the engine  at  the  desired  operating  speed,  then  turn  in  the needle    valve    until    the    engine    slows    down,    which indicates a lean mixture. Note the position of the needle valve,  then  turn  the  needle  valve  out  until  the  engine speeds  up  and  then  slows  down,  which  indicates  a  rich mixture, Note the position of the needle valve, then turn the  needle  valve  to  midway  between  the  lean  and  rich position.  Adjust the mixture to the requirement for each engine.      Remember   that   too   lean   a   mixture   is   not economical.      It   causes   overheating,   detonation,   and short valve life.  Also, since there is no accelerator pump, the  mixture  must  be  rich  enough  so  that  the  engine  will not stop when the throttle is suddenly opened.  Engines which run at constant speeds can be slightly leaner than those whose use requires changes in speed. THEORIES OF OPERATION Carburetion The  inset  of  Fig.    16  shows  what  happens  when  the needle  valve  is  turned  too  far.    A  square  shoulder  is produced on the taper.  It is possible, of course, to adjust the carburetor with the needle valve in this condition, but it  is  quite  difficult,  because  a  small   movement   of   the needle makes a big difference in the amount of fuel that can enter the nozzle.  And, if you do get it adjusted, the vibration can soon throw it off. Figure 17. To allow for different speeds, a flat disc called a butterfly, mounted  on  a  shaft,  is  placed  in  the  carburetor  throat above the venturi.   This  is  called  the  throttle.    See  Fig. 17. The throttle in the wide open position does not affect the air flow to any extent.  However, as the throttle starts to close,  it  restricts  the  flow  of  air  to  the  cylinder  and  this decreases  the  power  and  speed  of  the  engine.    At  the same  time  it  allows  the  pressure  in  the  area  below  the butterfly   to   increase.      This   means   that   the   difference between the air pressure in the carburetor bowl and the air pressure in the venturi is decreased, the movement of the  fuel  through  the  nozzle   is   slowed   down;   thus   the proportion   of   fuel   and   air   remain   approximately   the same.   As   the   engine   speed   slows   down   to   idle,   this situation changes.  See Fig.  18. At  idle  speed  the  throttle  is  practically  closed,  very  little air is passing through the venturi and the pressure in the venturi  and  in  the  float  bowl  are  about  the  same.      The fuel  is  not  forced  through  the’  discharge  holes,  and  the mixture tends to become too lean. Idle Valve To supply fuel for the idle, the nozzle is extended up into the idle valve chamber.  It fits snugly in the upper body to prevent leaks.  Because of this tight fit, the nozzle must be     removed     before     upper     and     lower     bodies     are separated, or the nozzle will be bent. 9 14