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: 

1. Ignition system 

2. Fuel-metering system 

3. Induction system 

4. Power section (valves, cylinders, etc.) 

5. Instrumentation 

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. 

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: 

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? 

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. Valves holding open or sticking open in one or more of the cylinders 

2. Lean mixture 

3. Intake pipe leakage 

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 

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 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 

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. 

Correct valve blow-by immediately to prevent valve failure and possible engine failure by taking the following steps: 

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. 

