There are several standards against which engine performance  can be compared. The performance of a particular engine can  be compared with its past performance, provided adequate  records are kept. Engine performance can be compared with  that of other engines on the same aircraft or aircraft having  identical installations. If a fault does exist, it may be assumed that the trouble lies  in one of the following systems:  Ignition system reasons for backfiring might be a cracked  distributor block or a high-tension leak between two ignition  leads. Either of these conditions could cause the charge in  the cylinder to be ignited during the intake stroke. Ignition  system troubles involving backfiring normally are not  centered in the basic magneto, since a failure of the basic  magneto would result in the engine not running, or it would  run well at low speeds but cut out at high speeds. On the other  hand, replacement of the magneto would correct a difficulty  caused by a cracked distributor where the distributor is a part  of the magneto. If a logical approach to the problem is taken and the  instrument readings properly utilized, the malfunctioning  system can be pinpointed, and the specific problem in the  defective system can be singled out. If the fuel system, ignition system, and induction system are  functioning properly, the engine should produce the correct  bhp unless some fault exists in the basic power section. Valve blow-by is indicated by a hissing or whistle when  pulling the propeller through prior to starting the engine,  when turning the engine with the starter, or when running  and blow-by past the intake valve is audible through  the carburetor. The more information available about any particular problem,  the better the opportunity for a rapid repair. Information that  is of value in locating a malfunction includes: Correct valve blow-by immediately to prevent valve failure  and possible engine failure by taking the following steps: From a different point of view, the powerplant is, in reality,  a number of small engines turning a common crankshaft  and being operated by two common phases: fuel metering  and ignition. When backfiring, low power output or other  powerplant difficulty is encountered, first find out which  system, fuel metering or ignition, is involved and then  determine whether the entire engine or only one cylinder  is at fault. For example, backfiring normally is caused by: 1.  Ignition system 2.  Fuel-metering system 3.  Induction system 4.  Power section (valves, cylinders, etc.) 5. Instrumentation 4.  An error in valve adjustment that causes individual  cylinders to receive too small a charge or one too large,  even though the mixture to the cylinders has the same  fuel/air ratio 1.  Was any roughness noted? Under what conditions of  operation? 2.  What is the time on the engine and spark plugs? How  long since last inspection? 3.  Was the ignition system operational check and power  check normal? 4.  When did the trouble first appear? 5.  Was backfiring or afterfiring present? 6.  Was the full throttle performance normal? 1.  Valves holding open or sticking open in one or more  of the cylinders 2.  Lean mixture 3.  Intake pipe leakage 1.  Perform a cylinder compression test to locate the  faulty cylinder. 2.  Check the valve clearance on the affected cylinder.  If the valve clearance is incorrect, the valve may be  sticking in the valve guide. To release the sticking  valve, place a fiber drift on the rocker arm immediately  over the valve stem and strike the drift several times  with a mallet. Sufficient hand pressure should be  exerted on the fiber drift to remove any space between  the rocker arm and the valve stem prior to hitting  the drift. 3.  If the valve is not sticking and the valve clearance is  incorrect, adjust it as necessary. 4.  Determine whether blow-by has been eliminated by  again pulling the engine through by hand or turning  it with the starter. If blow-by is still present, it may  be necessary to replace the cylinder. Valve Blow-By 10-39