will  lead  to  excessive  leakage.  Water  will  erode  the orifices of injection nozzles until they will not spray the fuel properly, thus preventing proper atomization. When this occurs, incomplete combustion and engine knocks result. Air in the fuel system is another possible trouble that may prevent an engine from starting. Even if the engine will start, air in the fuel system will cause the engine to miss and knock, and perhaps to stall. When an engine fails to operate, stalls, misfires, or knocks, there may be air in the high-pressure pumps and lines. In many systems, the expansion and compression of such air may take place even if the injection valves do not open. If this occurs, the pump is AIRBOUND. To determine if there is air in a fuel system, bleed a small amount of fuel from the top of the fuel filter; if the fuel appears quite cloudy, there are probably small bubbles of air in the fuel. Insufficient Fuel Supply An   insufficient   fuel   supply   may   result   from   a defective or inoperative part in the system. Such items as a closed inlet valve in the fuel piping or an empty supply tank are more likely to be the fault of the operator than of the equipment. But an empty tank may be caused by leakage, either in the lines or in the tank LEAKAGE.-You  can  usually  trace  leakage  in  the low-pressure  lines  of  a  fuel  system  to  cracks  in  the piping.  Usually  these  cracks  occur  on  threaded  pipe joints at the root of the threads. Such breakage is caused by   the   inability   of   the   nipples   and   pipe   joints   to withstand shock, vibration, and strains resulting from the  relative  motion  between  smaller  pipes  and  the equipment to which they are attached. Metal  fatigue  can  also  cause  breakage.  Each  system should have a systematic inspection of its fittings and piping  to  determine  if  all  the  parts  are  satisfactorily supported  and  sufficiently  strong.  In  some  instances, nipples may be connected to relatively heavy parts, such as valves and strainers, which are free to vibrate. Since vibration contributes materially to the fatigue of nipples, rigid  bracing  should  be  installed.  When  practicable, bracing should be secured to the unit itself, instead of to the  hull  or  other  equipment. Breakage   can   also   cause   leakage   in   the high-pressure  lines  of  a  fuel  system.  The  breakage usually occurs on either of the two end fittings of a line and is caused by lack of proper supports or by excessive nozzle opening pressure. Supports are usually supplied with an engine and should not be discarded. Excessive opening pressure of a nozzle-generally due to improper spring  adjustment  or  to  clogged  nozzle  orifices-may rupture  the  high-pressure  fuel  lines.  A  faulty  nozzle usually requires removal, inspection, and repair plus the use of a nozzle tester. Leakage  from  fuel  lines  may  also  be  caused  by improper replacement or repairs. When a replacement is necessary, always use a line of the same length and diameter as the one you remove. Varying the length and diameter  of  a  high-pressure  fuel  line  will  change  the injection characteristics of the injection nozzle. In   an   emergency,   you   can   usually   repair   a high-pressure fuel line by silver soldering a new fitting to the line. After making the silver solder repair, test the line for leaks and be certain no restrictions exist. Most leakage trouble occurs in the fuel lines, but leaks may occasionally develop in the fuel tank. These leaks  must  be  eliminated  immediately  because  of potential  fire  hazard. The  principal  causes  of  fuel  tank  leakage  are improper  welds  and  metal  fatigue.  Metal  fatigue  is usually  the  result  of  inadequate  support;  excessive stresses develop in the tank and cause cracks. CLOGGED   FUEL   FILTERS-Another   problem that can limit the fuel supply to such an extent that an engine will not start is clogged fuel filters. Definite rules for  filter  replacement  cannot  be  established  for  all engines. But instructions generally state that elements will not be used longer than a specified time. Since there are  reasons  that  an  element  may  not  always  function properly  for  its  expected  service  life,  it  should  be replaced whenever it is suspected of being clogged. Filter  elements  may  become  clogged  because  of dirty fuel, too small filter capacity, failure to drain the filter  sump,  and  failure  to  use  the  primary  strainer. Usually,  clogging  is  indicated  by  such  symptoms  as stoppage of fuel flow, increase in pressure drop across the filter, increase in pressure upstream of the filter, or excessive accumulation of dirt on the element (observed when the filter is removed for inspection). Symptoms of clogged filters vary in different installations, and each installation  should  be  studied  for  external  symptoms, such  as  abnormal  instrument  indications  and  engine operation. If external indications are not apparent, visual inspection of the element will be necessary, especially if it is known or suspected that dirty fuel is being used. Fuel filter capacity should at least equal fuel supply pump capacity. A filter with a small capacity clogs more 3-28