4• Clean the four fuel strainers after every  120 hours flying time, and oftener, if  necessary.  The strainers should be cleaned  immediately when irregular operation of either  engine develops.  5. Make certain that pressure in fuel system does  not exceed 25 lbs. per sq.in. when filling  from external pump.  6. Do not feed engines from a tank containing  less than 12 gallons of fuel as the outlet  is nearly uncovered with this amount of fuel  still in the tapk, and with the ship at a  30°  angle of pitch.  For the location of the strainers and  instructions .for cleaning them in flight,  see Fig. XXVII on the following page.  The various operations which can be performed with  the system, together with the proper settings of the valves,  are described in Figures XXVIII-a thru XXVIII-j for K-3  thru K-10 airships, and in Figures XXIX-a thru XXIX-j for  K-11 and future.  a  Run engine at 400 RPM.  b  Cylinder head temperature should be normal.  c  Have good plugs in engine.  d  Have mixture control FULL RICH.  e  Note RPM.  f  Rapidly move mixture control from FULL RICH to  IDLE CUT OFF positfon.  Catch the engine  before it stalls.  (g) Note RPM.  1. If large gain in RPM - Carburetor is set too  rich.  2. If loss in RPM - Carburetor is set too lean.  3.  Desired - gain of from 0 to 10 RPM.  The desired setting is the RICH BEST POWER setting  for the carburetor at the idle speed of the engine.  -70- C. RECOMMENDED CARBURETOR ADJUSTMENT 