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Ag Engineering Sample Lesson
AE263 Small Gasoline Engines: Engine Troubleshooting
MAIN IDEA: How do you determine the problem when an engine does not operate properly? Even with the best maintenance, engines will occasionally malfunction. Locating the problem area is the key to efficient, effective repairs of the engine. The problem can almost always be isolated to one of three areas: ignition, fuel, or compression. If all of these systems are functioning properly, the engine should run. However, if any one of these three systems is not functioning properly the engine will not run or will operate very poorly. A systematic check of these three systems is essential in order to locate and repair the problem. 1. THE IGNITION SYSTEM Several problems can occur with the ignition system, so a logical sequence of checks is needed. First, make sure the spark plug wire is correctly in place.
Check wires running to all kill switches to make sure there are no bare spots and that the wire is not grounding improperly. If a good strong spark is occurring, the armature is producing electricity and it is getting to the plug. However the timing of the spark may be off. The system is generated by a magnet on the fly wheel passing by the armature. If the magnet is not passing by at the correct time, the timing will be off.
To check the key, first remove the shroud from the engine. This usually can be accomplished by removing several small bolts. After the shroud has been removed, take off the screen and remove the flywheel nut using the correct tools. Remember that some of the flywheel nuts have left-hand threads. You should then be able to see the key in the keyway and a sheared or partially sheared key should be apparent.
Be sure to replace the key with an exact replacement. Any variation will result in poor performance or damage to the engine. 2. THE FUEL SYSTEM Once the electrical system is ruled out, begin a check of the fuel system. Begin by checking to make sure there is gas in the tank. Next remove the spark plug after attempting to start the engine several times. Check the end of the plug for gasoline. If the odor of gasoline is present on the plug or in the cylinder, the fuel system is probably working correctly.
Another problem might be that of flooding. Flooding refers to an excessive amount of gasoline in the cylinder. Such a condition may be caused by applying the choke too long. A more difficult cause to correct is the float needle valve sticking in the carburetor. If this is the case, the carburetor must be disassembled and corrections made. 3. THE COMPRESSION SYSTEM Compression is checked by the use of a compression tester. If the system is not building sufficient compression, the engine will malfunction. If the compression system is weak, a decision must be made whether to overhaul the engine or to replace it. Most of the less expensive small engines probably should be replaced rather than overhauling the entire engine. The most difficult system to repair is the compression system. It contains many parts, and most are on the inside of the engine. Repairs require a disassembly of the engine to get to the compression system parts. Because the block is made of aluminum, the cylinders may eventually wear to the point where repairs are ineffective. However, if the engine is expensive and the cylinders do not have too much wear, then an overhaul may be feasible. EXERCISES: 1. Locate a small engine that is not functioning properly and bring it to class. In the lab, go through a systematic process of determining the cause of the engine malfunction. List the steps you went through. 2. Once you have located the problem, determine the cost of repairing the engine. Make a list of replacement parts needed and call a parts dealer for prices. Decide whether the engine should be repaired or replaced. INTERNET RESOURCES:
** Briggs & Stratton
** Castrol Motor Oils
** Kohler Engines
** Lawn-Boy
** Poulan, Poulan Pro and Weed Eater
** Tecumseh TEST:
1. List the three areas where problems can occur in a small engine. 2. If a flywheel key is partially sheared, an engine will still run properly. TRUE or FALSE? 3. What is the first step in checking the fuel system? 4. Remove the __________ __________ to determine if the smell of gasoline is present. 5. Engine blocks can wear out beyond repair. They are usually made of __________ .
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AE263 Small Gasoline Engines: Engine Troubleshooting
TEACHER'S GUIDEOBJECTIVE: Students should be able to diagnose the cause(s) of engine failure. PREPARATION: Ask the students to bring in engines that have problems. Caution them not to bring in engines that have not run for a long time. These generally are not repairable. Before the students go to the lab for diagnosing the trouble, make sure they have been through the safety lesson for this section, AE264 Small Gasoline Engines: Safety. REFERENCE: Cooper, Elmer and Ray V. Herren. Agricultural Mechanics: Fundamentals & Applications, 4th ed. Albany, NY: Delmar Publishers, 2002. INTERNET RESOURCES:
** Briggs & Stratton
** Castrol Motor Oils
** Kohler Engines
** Lawn-Boy
** Poulan, Poulan Pro and Weed Eater
** Tecumseh IMPORTANT TERMS: ignition, fuel, compression, flywheel, flywheel key, magneto, spark plug, flooding, engine block. EXERCISE ANSWERS: 1. Look over each engine as it is brought in to make sure the engines have been operated within the past year and are in good enough condition to repair. If enough engines cannot be found, then use properly running engines and create problems such as a sheared flywheel key or a bare kill switch. 2. Locate shops that stock small engine parts and get price lists. Help the students decide whether or not to fix the engine. Make sure the student understands that he or she is responsible for the cost of the parts. TEST KEY:
1. List the three areas where problems can occur in a small engine. 2. If a flywheel key is partially sheared, an engine will still run properly. TRUE or FALSE? FALSE - even if a key is only partially sheared, the timing will still be off. 3. What is the first step in checking the fuel system? Check to see if there is fuel in the tank. 4. Remove the spark plug to determine if the smell of gasoline is present.
5. Engine blocks can wear out beyond repair. They are usually made of aluminum.
END TEACHER'S GUIDE
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Next, check the spark output by using a spark tester connected between the spark plug and spark plug wire. If there is no spark or a weak spark, the problem is probably either a bad armature or a grounded wire.
The flywheel is generally held in correct position with the crankshaft by a soft metal key in the keyway. This assures correct engine timing. The key will shear off if the engine is subjected to an abrupt stop such as what would occur if a mower blade hit an object. The shearing of a key decreases the chances of crankshaft breakage. If the key is only partially sheared, the timing will still be off.
To replace the key it is necessary to remove the flywheel. Appropriate pullers are needed to remove flywheels without breakage. To pull the flywheel, remove the crankshaft nut, install the puller, tighten the puller to exert pulling force, and tap the center bolt of the puller with a light blow using a steel hammer. The impact should cause the wheel to pop off of the tapered shaft.
If the odor of gasoline is not present, trace the flow of fuel from the tank to the carburetor and look for fuel line blockages or restrictions. Also, there may be a tear in the diaphragm in the carburetor. This would prevent the gasoline from being lifted from the tank to the carburetor. Replace the diaphragm or the entire carburetor.
