DISTORTION Warpage  or  distortion  of  cylinder  heads  becomes apparent when the mating surfaces of the head and block fail to match properly. If distortion is severe, the head will not lit over the studs. Distortion may be caused by improper welding of cracks or by improper tightening of the cylinder head studs. Occasionally, new heads may be  warped  because  of  improper  casting  or  machining processes. Repair  of  distorted  or  damaged  cylinder  heads  is often impracticable. They should be replaced as soon as possible  and  turned  in  to  the  nearest  supply  activity, which  will  determine  the  extent  of  damage  and  the method of repair. FOULING If  the  combustion  chambers  become  fouled,  the efficiency  of  combustion  will  decrease.  Combustion chambers are designed to create the desired turbulence for  mixing  the  fuel  and  air;  any  accumulation  of  carbon deposits  in  the  space  will  impair  both  turbulence  and combustion  by  altering  the  shape  and  decreasing  the volume  of  the  combustion  chamber. Symptoms of fouling in the combustion chambers are smoky exhaust, loss of power, or high compression. Such  symptoms  may  indicate  the  existence  of  extensive carbon formation or clogged passages. In some engines, these symptoms indicate that the shutoff valves for the auxiliary combustion chambers are stuck Combustion  chambers  may  also  become  fouled because   of   faulty   injection   equipment,   improper assembly procedures, or excessive oil pumping. Cleaning  of  fouled  combustion  spaces  generally involves removing the carbon accumulation. The best method is to soak the dirty parts in an approved solvent and then wipe off all traces of carbon. You may use a scraper  to  remove  carbon,  but  be  careful  to  avoid damaging the surfaces. If oil pumping is the cause of carbon formation, check the wear of the rings, bearings, pistons, and liners. Replace or recondition excessively worn parts. Carbon formation resulting from improperly assembled parts can be avoided by following procedures described in the manufacturer’s technical manual. INSPECTING, TESTING, AND REPAIRING VALVES AND VALVE ASSEMBLIES Regardless   of   differences   existing   in   engine construction, there are certain troubles common to all assemblies. STICKING  VALVES Sticking valves will produce unusual noise at the cam  followers,  pushrods,  and  rocker  arms  and  may cause the engine to misfire. Sticking is usually caused by resinous deposits left by improper lube oil or fuel. To  free  sticking  valves  without  having  to disassemble  the  engine,  use  one  of  several  approved commercial  solvents.  If  the  engine  is  disassembled,  use either a commercial solvent or a mixture of half lube oil and half kerosene to remove the resins. Do NOT use the kerosene mixture on an assembled engine, since a small amount of this mixture settling in a cylinder could cause a  serious  explosion. BENT VALVES Bent or slightly warped valves tend to hang open. A valve  that  hangs  open  not  only  prevents  the  cylinder from firing, but also is likely to be struck by the piston and bent so that it cannot seat properly. Symptoms of warped or slightly bent valves will usually show up as damage to the surface of the valve head. To lessen the possibility  that  cylinder  head  valves  will  be  bent  or damaged  during  overhaul,  NEVER  place  a  cylinder head directly on a steel deck or grating; use a protective material such as wood or cardboard. Also, NEVER pry a valve open with a screwdriver or similar tool. WEAK SPRINGS Valves may close slowly, or fail to close completely, because of weak springs. At high speeds, valves may “float,” thus reducing engine efficiency. Valve springs wear quickly when exposed to excessive temperatures and to corrosion from moisture combining with sulfur present in the fuel. BURNED VALVES Burned valves are indicated by irregular exhaust gas temperatures  and  sometimes  by  excessive  noise.  In general, the principal causes of burned valves are carbon deposits, insufficient tappet clearance, defective valve seats,  and  valve  heads  that  have  been  excessively reground. 3-8