Troubleshooting Small Engine
Systems
Jason Spurlin
4/10/02
Virginia Tech
Edited by the CTAE Resource Network February 2011
Troubleshooting Engine Systems
PERFORMANCE OBJECTIVE
On a lab project, perform systematic troubleshooting
of the electrical, lubrication, cooling, fuel, and
compression systems with at least 70% accuracy.
ENABLING OBJECTIVES
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Identify symptoms of malfunctioning system components.
Inspect spark plug, flywheel key, and magneto.
Inspect and repair components of lubricating system.
Inspect, clean, and repair cooling fins, flywheel, and air shroud.
Inspect fuel tank, fuel pump, carburetor, reed valves (in twocycles), fuel lines, filters, and air cleaner.
Inspect governor and adjust if necessary.
Identify and repair causes of poor compression.
Ignition System
Engine won’t Start
Cause
Remedy
Disconnected spark plug wire
No spark at plug
Spark plug porcelain cracked
Spark plug fouled
Faulty magneto
Sheered flywheel key
Connect wire
check switches and magneto
replace plug
clean and regap
check air gap, if necessary overhaul mag.
replace flywheel key
Engine Misses under Load
Fouled Plug
Porcelain cracked
Improper spark plug gap
Pitted breaker points
Faulty condensor
clean and regap
replace plug
regap
replace points
check on a tester, replace if necessary
Ignition System cont.
Engine Lacks Power
Magneto improperly timed
reset air gap
Engine Runs Unevenly
Intermittent spark at plug
Other ignition problems:
•
rust on iron plates
•
faulty magnets
check switches, check magneto for loose or
damaged wires
Armature air gap too large
Disconnected spark plug wire
Burned plug
Lubrication System
Lubrication System
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Lack of Lubrication Causes:
– Engine to miss under load
Fill crankcase to
– Lack of power
proper level and/or
check oil slinger
– Overheating
Engine uses excessive oil
– oil level too high
Drain oil to proper level
– oil filler cap loose of gasket damaged > replace gasket
– oil passages obstructed > clean oil passages
Cooling System
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A malfunctioning cooling system will cause
the engine to overheat
 When checking cooling system look for:
– obstructed air flow
– clogged cooling fins
– broken cooling fins
– broken flywheel vanes
– damaged or removed shroud
– low oil level
Broken air fin
Clogged air passages
Fuel System
Fuel System
Fuel system has to deliver clean, fresh, and properly proportioned
fuel/air mixture to combustion chamber.
Engine Fails to Start
Cause
No fuel in tank
Shut-off valve closed
Obstructed fuel line
Tank cap vent obstructed
Water in fuel
Overchoked
Improper carburetor adjustment
Primer bulb busted
Remedy
Fill tank
Open valve
Remove and clean fuel line
Open vent
Drain tank and refill with clean fuel
Close shut off valve and pull cord until
engine starts
Adjust carburetor
Replace primer bulb
Fuel System cont.
Engine Misses Under Load
Improper carburetor adjustment
Reed valves stuck
Adjust carburetor
Clean or replace reed
Engine Lacks Power
Choke partially closed
Improper carburetor adjustment
Air cleaner dirty
Reed fouled (2 cycle)
Improper fuel/oil mixture (2 cycle)
Open choke
Adjust carburetor
Clean or replace air filter
Clean or replace
Drain tank; fill with proper mixture
Engine Surges or Runs Unevenly
Fuel tank cap vent hole clogged
Governor parts sticking or binding
Carburetor throttle linkage or throttle shaft
binding or sticking
Improper carburetor adjustment
Dirty Carburetor
Open vent hole
Clean and repair if necessary
Clean, lubricate, or adjust linkage
Adjust carb
Clean carburetor
Intact Primer Bulb
Clean Air Filter
Clean Carburetor
Fuel Line
Compression System
Compression System
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The following are symptoms of poor compression:
– Engine will not start
– Engine knocks
– Engine misses under load
– Engine lacks power
– Engine uses excessive oil
Common causes of poor compression.
– Damaged or removed crankcase gaskets
– Carbon deposits in combustion chamber
– Weak valve springs
– Leaking valves
– Cracked porcelain on spark plug
– Excessive cylinder and piston wear
Burnt valve
Chipped piston
Missing head gasket
Checking Compression
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An engine with proper compression will have a
degree of resistance when the operator pulls on the
starter rope
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Use a compression tester
– 30-45 psi min. for starting
– 90 psi min. for efficient operation
Other Ailments
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Clogged exhaust system
– Insects frequently build nests in
exhaust pipes
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Associated equipment
malfunctioning
Summary
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Once you have checked the easy things, work through
each system until you find the problem.
Faulty ignition systems can be eliminated if there is spark
at the plug. If not, focus attention on magneto,
connecting wires, plug, and the flywheel magnets.
A faulty cooling system will cause the engine to overheat
and stall, but cooling problems can be easily alleviated by
making sure air passages and cooling fins are clean and
intact.
Summary cont.
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The fuel system has to allow a CLEAN, proper fuel/air
mixture to reach the combustion chamber. Make sure that
fuel lines and filters are clear and that the carburetor is
adjusted properly.
Poor compression is easily identified by resistance when rope
starting and by using a compression tester. Problems are
generally associated with leaky gaskets, leaking valves, and
excessive cylinder wear.
Clogged exhaust systems also prohibit an engine from
running properly.
REMEMBER: Check off each thing that you have checked
and/or repaired.