Warner Engine Handbook:

Engine Troubles and Service Repair

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Section VI - Engine Troubles and Service Repair

1. Failure of Engine To Start

a. Inadequate Fuel Supply

1. See that gasoline supply is "on".

2. Check to see that the gasoline in the tank is high enough to flow to the carburetor.

3. Make sure that the carburetor float is not stuck and that there is a definite flow of gasoline to the carburetor.


In no case will the carburetor be disassembled to determine these conditions.

4. Check fuel lines and tank for possibility of vapor lock.

5. Check fuel lines to see if flow is restricted by obstructions such as dirty strainers or water in lines.

b. Engine Under-Primed or Over-Primed

1. The correct amount of priming necessary for each engine must be determined by trial.

2. It engine is under-primed, check functioning of primer system and "load" the engine more by turning the propeller in the direction of rotation with the switch "off" and throttle "closed".

3. Over-priming and a resultant flooded condition is more often experienced during warm or hot weather.

4. Over-priming is easily identified, especially it the engine seems to fire on one or two cylinders with a fizzling exhaust report and emission of black smoke accompanied by a weak "kick" of the propeller with the engine finally dying.

5. If over-prim1ng is indicated, turn the main gasoline supply valve "off"; turn the magneto switch "off"; open the throttle wide and turn the propeller opposite direction of rotation ten to fifteen revolutions. This should "unload" engine and a new attempt can be made to start.

c. Mixture Control and Throttle Operation

See section on Starting and Normal Operation.

d. Defective Ignition

1. Examine ignition wires for shorts or leaks caused by faulty insulation.

2. Make sure that the ignition switch is turned on, is not defective, or that the ground wire insulation is not damaged so as to permit a contact with the metal of the airplane somewhere between the switch and magneto ground terminals.

3. Check magneto points for proper gap clearance, a possible pitted condition, or evidence that the condenser has burned out. Refer to the section on Limits for Proper Gap.

4. Ascertain that the magnetos are in good working condition and in correct time. Refer to section on timing of the engine for this information.

5. Check all spark plugs for correct gap setting and see that they are not fouled by oil or carbon. A visual examination may reveal the cause of failure.

(a) If the plug is clean but the metal snows signs of overheating then the cylinder has been running too hot, which indicates pre-ignition, detonation or poor cooling.

(b) If the plug is clean but wet with fuel or oil, no conclusions should be formed since this may occur during the moment of stopping only.

(c) Caked carbon is evidence of the beginning of excessive oil consumption or misfiring or non-firing of the plug.

(d) A thin deposit of fine black carbon may be sufficient to short circuit a plug which otherwise looks quite free from deposits.

e. Valve Action

1. Check valve stems to be sure that they are not gummed up with carbon making the valve stick open.

2. See that all valve springs are in good order and that their assembly to the valve stem is in good order.

3. Check to ascertain the free and good working order of all rocker arm assemblies, especially evidence of a rocker arm striking the side of a rocker box causing a delayed action.

4. Check for worn or bent pushrods.

5. Check for correct valve clearance.

f. Cold Oil

Turn propeller over by hand to break the drag created by cold oil. If engine is excessively stiff, it may be necessary to drain and heat the oil.

g. Hot Engine

A hot engine is exceedingly easy to over-prime, and it must be appreciated that frequently it will start without an additional prime over that left in the cylinders when the engine was shut down. If the engine is hot and seems to refuse to start for that reason, turn main gasoline supply off; open throttle, turn ignition switch off, and allow engine to stand for ten or fifteen minutes and cool. Do not operate engine at any time without all cowling and baffles installed. After engine has cooled, proceed as directed for over-primed engine.

h. Air Leaks In Induction System

Examine intake pipes for leaks at all induction system connections. Air leaks sometimes cause a sharp high-pitched whistling noise that is particularly audible at or near idling speeds when the intake manifold vacuum is at its highest.

i. Carburetor Flooding


Because of the fire hazard involved, the engine should not be started if the carburetor leaks excessively.

1. If there is a slight leakage, it may be due to sticking of the float. A light tap on the carburetor housing with the hand w111 usually remedy this condition.

2. In no case will the carburetor be disassembled to correct any leakage. Replace the carburetor instead or remove it and repair it. Refer to the carburetor manual.

2. Low Oil Pressure

a. Determine whether main oil tank contains a sufficient quantity or lubricating oil.

b. Carefully inspect main oil line from tank to input side or duplex oil pressure pump to ascertain if there are any leaks causing the pump to starve for oil.

c. During cold weather the oil may become congealed in the suction line from the oil tank, preventing the oil from reaching the pump.

d. Excessive temperatures reduce the viscosity or the oil, which then offers less resistance when forced through the oil passages and thus tends to make the pressure drop.

e. Foaming is a frequent cause of fluctuating oil pressure and loss of oil pressure. The presence of some air in the scavenge oil is normal. If the oil is foaming, it should be drained from the oil tank and replaced with fresh oil. Water or metal soaps in the oil will cause excessive foaming.

f. Inspect oil pressure relief valve to ascertain whether the ball is seating well, and that the control spring is functioning properly.

g. Ascertain whether oil pump is actually turning so that excessive wear of gears or housing or failure of drive or drive shaft has not occurred. Consult directions in overhaul section of handbook for repair of pump.

h. Remove oil pressure screen and observe if it is partly clogged or contains an excessive deposit of metal particles. If metal particles of appreciable size and quantity are found, the cause should be determined and the engine cleaned out and repaired if necessary before further operation.

3. Low Power

a. Remove front set of spark plugs and test compression of each cylinder. If no gauge is available, cylinder compression can be determined with a reasonable degree of accuracy by placing the thumb over the spark plug hole and swinging the propeller by hand so as to bring the piston up on compression stroke. At top of compression stroke, suddenly remove thumb from spark plug hole so as to rapidly release compression and a rather loud, snappy report of escaping air should be heard. If the propeller is rotated twice and the number of uniform compression strokes equals the number of cylinders, lack of or uneven compression could, in most cases, be eliminated as the cause of the trouble. A little practice will produce fairly accurate results. Blown or sticking valves, or excessive loss of compression past pistons can easily be heard by listening to the individual cylinder being tested.

b. Check all valves. Any valve having appreciable increased clearance will indicate cam ring, push rod, or rocker arm trouble. It is impossible for the engine to jump valve timing.

c. Ascertain whether engine lubricating oil is correct grade or not.

d. Check ignition system operation, especially for cylinders periodically "cutting out" due to failure of spark plugs, ignition wiring, or sticking of magneto points. Be sure magneto is functioning in accordance with instructions set forth in the manufacturer's handbook. Ascertain that magneto breaker point has minimum clearance behind the arm when points themselves have their required clearance. Ascertain whether magnetos are in full advance. If necessary, check ignition timing in accordance with instructions in Section XI of this handbook.

e. Make sure that pilot's throttle lever has full operation and is completely opening the butterfly in the carburetor.

f. Check carburetor for proper setting. See Carburetor Manufacturer's handbook of instructions.

g. See that an unrestricted flow of gasoline of the proper grade is available at the carburetor.

h. Ascertain that carburetor air heater, if provided, is being operated properly and that there is no evidence of ice in the induction system.

i. Check for air leaks in the induction system.

j. Check propeller for weight and setting.

4. Rough Running

a. Check the propeller for balance, track, and correct installation on the propeller shaft.

b. Remove and check spark plugs.

c. Check magneto operation. Ascertain whether insulation of ignition cables is failing at high engine speeds.

d. Check valve operation, especially evidence of sticking or any lag in valve operating mechanism. Check valve clearance.

e. Check engine mounting bolts for looseness.

f. Check engine mount for cracked or broken members.

g. In extreme cases, check possibility of magneto ground wire swinging and periodically grounding, or having damaged insulation at the points where they are taped down.

h. If airplane has been nosed over, check for bent crankshaft.

i. See that propeller hub nut is tight.

j. Check air intake temperature to intake sure that it is not too hot.

k. Check for ice formation in the induction system.

l. Make sure that the engine operating temperature is not too low.

m. Check carburetor for proper mixture setting.

5. High Oil Temperature

a. Check quantity and quality of supply of oil in main oil tank. Oil may be too light or excessively diluted.

b. Check functioning of oil cooler and by-pass valves if airplane is so equipped.

c. Check compression as described in paragraph 3.a. in this section. Excessive discharge of oil or oil fumes at crankcase breather would indicate piston blow-by with excessive pressure, due probably to worn out piston rings.

d. Check to see that crankcase vent system is functioning properly.

e. Check scavenger pump for proper operation.

f. Check oil screen strainer for deposits indicating any evidence of failure in the master rod bearing.

g. Check air flow around oil reservoir tank.

6. High Cylinder Head Temperature

a. Check ignition timing. Engine time may be too late.

b. A lean mixture in any or all cylinders could cause high temperatures. This could result from improper setting of carburetor or air leaks in induction system.

c. Test for pre-ignition or detonation.

d. Check inter-cylinder baffling, being sure of its proper installation.


Inter-cylinder baffling should be tight against the O.D. of cylinder fins.

7. Carburetor

a. Idle adjustment, located on the front side of carburetor, is set for smoothest idle on final acceptance run of engine at the factory. As the engine loosens up during service the adjustment should be regulated as the needs require, to keep the engine idling smoothly.

b. Any further maintenance, service or adjustment will be accomplished in accordance with carburetor manufacturer's instructions.

8. Magneto

All adjustments or service of magnetos will be accomplished in accordance with the instructions published by the magneto manufacturer.

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