Troubleshooting Small Engines

1
Troubleshooting Small Engines
2
Objectives

• The Student Will Be Able To
• List, summarize, and employ the 5 troubleshooting
  rules
• Interpret common symptoms of problems that occur
  in small engines and
• Compile the information given in this laboratory
  section in order to diagnose and correct small
  engine problems throughout the remainder of the
  semester.

3
Troubleshooting

• The skills involved in troubleshooting engine
  problems are very similar to the skills used by
  medical doctor in diagnosing health problems. As
  a troubleshooting "doctor", students must know
  the causes of certain symptoms that your
  mechanical patient might have. Luckily
  mechanical medical school or a residency is not
  required before attempting to work on a patient.
  All that is needed are the 5 simple rules to
  troubleshooting small engines.

4

• The five troubleshooting rules should be followed
  in sequence in order that the maximum amount of
  time and money be saved. These five rules are
  also very general this allows them to work for
  just about all makes and models of small engines.

5
Remember these 5 Rules for troubleshooting engine
problems
Please Excuse My Dear Aunt Sally

• Spark
• Compression
• Accessories
• Carburetion
• Timing

6
Rule 1 Spark

• Spark refers to the high voltage spark that is
  created by the ignition system. Without spark
  the engine is completely unable to function.
  Checking the ignition for "Spark" is the first
  rule because of its simplicity.
• To check to see if the ignition system is
  functioning properly a spark tester is used. As
  mentioned is Laboratory II there are numerous
  types and ways to test for "Spark", and they are
  all effective. If the ignition system is
  creating an adequate "Spark", but still does not
  start, continue on to Rule 2. If the ignition
  system fails to create a "Spark", check this list
  of items for possible solutions.
• http//www.youtube.com/watch?vqAVz6ICOkKU

7
Possible Spark Solutions

• Courtesy of Briggs Stratton Corp.
• 1. Check the spark plug, it may be faulty.
• 2. Check the armature air gap. (Figure 6.0)
• 3. Check to see if the flywheel key is sheared.
  (Figure, 6.1 and 6.2)
• 4. Check for an incorrect breaker point gap.
• 5. Check for dirty or burned breaker points.
• 6. Check for a worn or stuck breaker plunger.
• 7. Check for a shorted ground wire or stop
  switch.
• 8. Check for condenser failure.
• 9. Check for armature failure.
• 10. Check for worn bearings and/or shaft on the
  flywheel side.

Fig. 6.1, Flywheel Keys
Fig. 6.2, Sheared Flywheel Key
Fig. 6.0, Armature Flywheel
8
Magnetron Ignition Systems

• Starting in 1982, all Briggs Stratton engines
  were equipped with Magnetron ignition armatures,
  leaving Breaker Points and Condensers by the
  wayside. The Magnetron ignitions are much more
  reliable, but if you still fail to get a Spark
  you might check
• 1. To see if the Stop Switch is grounded.
• 2. The interlocking system, if equipped.
• 3. Armature air gap too wide.
• 4. Ignition armature ground wire eyelet
  disconnected.
• 5. Broken external wires at the ignition
  armature.
• 6. Blown ignition armature from 12 volt battery
  current.

9
Rule 2 Compression

• A great deal of compression is needed in the
  cylinder in order that the air-fuel mixture
  ignites properly. An engine with little
  compression may drastically lose power or may
  fail to start at all. Compression is the second
  rule because it is almost as quick and simple as
  checking for Spark.
• Check for Compression in one of two ways. The
  first method involves removing the spark plug and
  inserting a compression gauge in its place, in
  order to measure how many psi the piston exerts
  on the cylinder head. With the gauge threaded
  into the cylinder head the starter rope should be
  pulled two to three times. If the engine has a
  starter, the engine should be turned over for
  about two to three seconds. Either of these
  should give a fairly accurate reading on the
  gauge. The second method does not require you to
  remove the spark plug. All that a person needs
  to do to determine compression using the second
  method is to spin the flywheel clockwise. If the
  flywheel rebounds sharply the engine is
  considered to have adequate compression for
  ignition.
• If the engine has adequate compression, but
  still fails to start, move on to Rule 3. If the
  engine does not have adequate compression the
  following list of check points should be
  considered.

10
If the engine does not have adequate compression
the following list of check points should be
considered.

• Check For
• 1. A loose spark plug
• 2. Loose cylinder head bolts
• 3. A blown head gasket
• 4. Burnt valves and/or seats
• 5. Insufficient tappet clearance
• 6. A warped cylinder head
• 7. Warped valve stems
• 8. Worn or scared cylinder walls
• 9. Worn or broken piston rings
• 10. Broken connecting rod

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Rule 3 Accessories

• Accessories are all of those mechanisms that
  control, or are controlled by the engine. Many
  times faulty, broken, or jammed accessories such
  as belts, pulleys, electrical equipment, blades,
  controls, etc.. may be the cause of the engines
  failure to start. A thorough check of all
  accessory equipment can solve many starting
  problems.
• If the engine has no accessory problems, but
  still fails to start, continue on to Rule 4. If
  a problem is found, it should be corrected and
  periodically checked for reoccurrences.

12
Rule 4 Carburetion

• The carburetor is a complex part of the small
  engine that might have several reasons for
  causing starting difficulty.
• Before disassembling the carburetor, check for
  the obvious! Check the fuel line, the fuel
  filter, the fuel tank, and the fuel itself for
  water or contaminants. Any of these may restrict
  ignition. If the fuel contains water, drain all
  liquid from the tank and disassemble the
  carburetors fuel pump or bowl and clean now
  reassemble the carburetor and fill the tank with
  fresh fuel. Many times a quick check of these
  items will solve the problem and save you work.
• Basic carburetor adjustments are given below.
  The needle valve seats are made of soft brass and
  can be easily damaged. When turning the needle
  valve in, take care not to use force because you
  may damage the needle valve and the seat, (See
  Figure, 6.3). Loosening the packing nut will
  allow the screw to turn with greater ease. If
  fuel seeps from around the adjustment screw,
  tighten the packing nut.

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• BASIC CARBURETOR ADJUSTMENT

• FINAL ADJUSTMENT

• Place the throttle control in the fast
  position. Turn the needle valve clockwise until
  the engine just starts to slow. Now open the
  needle valve 3/8 of a turn counterclockwise.
  Then rotate the throttle arm counterclockwise and
  hold it against the throttle stop while adjusting
  the idle RPM by turning the idle speed adjusting
  screw to obtain 1750 RPM. Release the throttle
  and the engine should accelerate smoothly. If it
  doesnt, the carburetor should be re-adjusted,
  usually to a slightly richer mixture by turning
  the needle valve counterclockwise 1/8 turn more.
  (See Figure, 6.4).
• Fig. 6.4
• The most common carburetor problems related to
  starting can be corrected by disassembling,
  cleaning, adjustment, and replacing old
  gaskets/diaphragms in the carburetor. This is a
  shotgun approach, cure-all for correcting
  carburetor problems. Other carburetor problems
  may be caused by
• 1. Over choking.
• 2. Excessively rich fuel mixture.
• 3. Valve malfunction.
• 4. Faulty floats or springs.
• 5. Leaking gaskets.
• 6. Broken or clogged inlet tubes.
• 7. Broken needle valve assembly.
• 8. Fuel tank on/off valve, turned off.
• If you have completed work on the carburetor and
  the engine still fails to start, move to Rule 5!

• Gently turn the needle valve until it just
  closes. The valve may be damaged if it is closed
  tightly. Next, open the needle valve 1-1/2 turns
  counter-clockwise. The initial adjustment will
  permit the engine to be started. Now warm-up the
  engine (approx. 5 minutes) in preparation for
  final adjustment.

14
Rule 5 Timing

• Checking and resetting the timing is the most
  time consuming troubleshooting step because it
  involves draining the engine oil and removing the
  crankcase cover, therefore, it is done last.
• If the timing is off, the engine will be out of
  synchronization, and will not start. The timing
  can be corrected by draining the oil, removing
  the crankcase cover, and realigning the timing
  dots that are marked on the crankshaft gear and
  the cam gear. (See Figure, 6.5 and 6.6).
  Finally, reassemble the engine and refill the
  crankcase with oil.

Fig. 6.5, Aligning Timing Marks
Fig. 6.6, Aligning Timing Marks
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• Note One of these Five Rules should correct
  the engines starting problem. Many times the
  engine will start, but problems persist. Such
  problems may cause poor or erratic engine
  performance. The following are some small
  engine trouble symptoms and their probable
  causes.

16

• Vibration
• Rotating parts out of balance.
• Bent crankshaft.
• Loose mounting bolts.
• Carburetor not properly adjusted.
• Mounting deck cracked.
• Runs Rough/Wont Accelerate Properly
• Incorrect spark plug gap.
• Engine not warm.
• Low fuel level.
• Idle set too low.
• Fuel mixture too lean.

17

• Backfires While Throttling Up or Down
• Water in fuel.
• Low fuel level.
• Fuel mixture too lean/rich.
• Choke malfunction.
• Dirty carburetor.
• Kill switch contacting.
• Leaking/worn valves.
• Dies at High Idle
• Low fuel level.
• Low oil level.
• Loose spark plug lead.
• Carburetor out of adjustment.

18

• Will Not Reach High Idle RPM
• Choke not disengaged completely.
• Throttle control loose or installed improperly.
• Clogged fuel filter.
• Fuel line too small.
• Low fuel level.
• Fuel tank too far from engine.
• Fuel pump malfunction.
• Gravity feed tank below carburetor.
• Trash in carburetor.
• Carburetor out of adjustment.
• Incorrect spark plug gap.
• Starts Dies Quickly
• Worn, torn, and/or faulty carburetor diaphragm.
• Low fuel level.
• Obstruction in fuel line or carburetor.
• Carburetor needs to be disassembled and cleaned.