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TECH TIPS

small engine Spark Plug Troubleshooting Guide

This guide provides an introduction into diagnostic procedures for 2- and 4-cycle engines. It examines the three primary areas necessary for proper operation of any engine: ignition, fuel delivery and mechanical functions.

Step 1: Check your ignition system.

Keep an eye for:

  • Faulty ignition switch
  • Broken, damaged or corroded wire
  • Timing
  • Improper spark plug gap
  • Faulty ignition coil
  • Faulty spark plug wire

Be sure to take a close look at your spark plug within the combustion chamber. It's an excellent tool for diagnosing engine problems. Though spark plugs themselves are rarely the cause of the problem, they do interact with fuel delivery, spark delivery and mechanical functions. Examination of the spark plugs can sometimes give a physical clue of the underlying cause.

Step 2: Ensure you have fuel, that it's not contaminated nor stale.

Make sure there's no water in the fuel tank and the octane rating isn't too low.

Step 3: Select your engine concern to understand related causes.

Misfires or "Skips"

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

Improper Oil or Fuel/Oil Ratios (2-cycle engine only)

  • Improper mix
  • Separation of oil and fuel
  • Main bearing seal leakage

Battery Condition

  • Low voltage
  • Inadequate charge
  • Internal short circuit
  • Corrosion
  • Poor or loose connections
  • Faulty starter
  • Lack of battery fluid

Exhaust Restriction

  • Damaged exhaust system
  • Clogged or restricted baffles or exhaust port

Will Not Start

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

No Spark

  • Ignition switch off
  • Wiring in primary or secondary circuit disconnected, broken or corroded
  • Contact breakers stuck or oxidized
  • Coil and condenser faulty
  • Battery condition
  • Ignition module - no signal
  • Poor or loose connections

Battery Condition

  • Low voltage
  • Inadequate charge
  • Internal short circuit
  • Corrosion
  • Poor or loose connections
  • Faulty starter
  • Lack of battery fluid

Low or Loss of Compression

  • Cracked or broken piston rings
  • Damaged head gasket or cracked cylinder/cylinder head
  • Valve sticking or broken push rod
  • Pre-ignition damage
  • Detonation damage

Valve Problems (4-cycle engine only)

  • Improper adjustment in lash
  • Improper seating due to burned, bent or stuck valve
  • Broken valve spring
  • Valve or valve seal leaking

Is Hard to Start

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

 Improper Oil or Fuel/Oil Ratio (2-cycle engine only)

  • Improper mix
  • Separation of oil and fuel
  • Main bearing seal leakage

Battery Condition

  • Low voltage
  • Inadequate charge
  • Internal short circuit
  • Corrosion
  • Poor or loose connections
  • Faulty starter
  • Lack of battery fluid

Low or Loss of Compression

  • Cracked or broken piston rings
  • Damaged head gasket or cracked cylinder/cylinder head
  • Valve sticking or broken push rod
  • Pre-ignition damage
  • Detonation damage

Exhaust Restriction

  • Damaged exhaust system
  • Clogged or restricted baffles or exhaust port

Valve Problems (4-cycle engine only)

  • Improper adjustment in lash
  • Improper seating due to burned, bent or stuck valve
  • Broken valve spring
  • Valve or valve seal leaking

Backfires

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

Valve Problems (4-cycle engine only)

  • Improper adjustment in lash
  • Improper seating due to burned, bent or stuck valve
  • Broken valve spring
  • Valve or valve seal leaking

Overheats

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

Oil or Fuel/Oil Ratio (2-cycle engine only)

  • Improper mix
  • Separation of oil and fuel
  • Main bearing seal leakage

Cooling (2-cycle engine only)

  • Restricted in cooling system or fins
  • Lack of coolant
  • Cooling fin damage
  • Damaged head gasket

Lubrication

  • Out of oil
  • Improper oil or fuel/oil ratio

Carbon Buildup (2-cycle engine only)

  • Normal accumulation
  • Excessive rich mixture, fuel/oil ratio
  • Incorrect oil type

Faulty Governor

  • Governor assembly malfunctioning
  • Linkage out of adjustment, worn or disconnected
  • Governor spring sensitivity too great
  • Linkage binding

Engine Overload

  • Engine operated in conditions above those for which designed

Valve Problems (4-cycle engine only)

  • Improper adjustment in lash
  • Improper seating due to burned, bent or stuck valve
  • Broken valve spring
  • Valve or valve seal leaking

Idles Poorly

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

Low or Loss of Compression

  • Cracked or broken piston rings
  • Damaged head gasket or cracked cylinder/cylinder head
  • Valve sticking or broken push rod
  • Pre-ignition damage
  • Detonation damage

Exhaust Restriction

  • Damaged exhaust system
  • Clogged or restricted baffles or exhaust port

Valve Problems (4-cycle engine only)

  • Improper adjustment in lash
  • Improper seating due to burned, bent or stuck valve
  • Broken valve spring
  • Valve or valve seal leaking

Stops Suddenly

Oil or Fuel/Oil Ratio (2-cycle engine only)

  • Improper mix
  • Separation of oil and fuel
  • Main bearing seal leakage

No Spark

  • Ignition switch off
  • Wiring in primary or secondary circuit disconnected, broken or corroded
  • Contact breakers stuck or oxidized
  • Coil and condenser faulty
  • Battery condition
  • Ignition module - no signal
  • Poor or loose connections

Cooling

  • Restricted in cooling system or fins
  • Lack of coolant
  • Cooling fin damage
  • Blown head gasket

Lubrication

  • Out of oil
  • Improper oil or fuel/oil ratio

Low or Loss of Compression

  • Cracked or broken piston rings
  • Damaged head gasket or cracked cylinder/cylinder head
    Valve sticking or broken push rod
  • Pre-ignition damage
  • Detonation damage

Valve Problems (4-cycle engine only)

  • Improper adjustment in lash
  • Improper seating due to burned, bent or stuck valve
  • Broken valve spring
  • Valve or valve seal leaking

Lacks Power

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

Improper Oil or Fuel/Oil Ratio (2-cycle engine only)

  • Improper mix
  • Separation of oil and fuel
  • Main bearing seal leakage

Cooling

  • Restricted in cooling system or fins
  • Lack of coolant
  • Cooling fin damage
  • Damanged head gasket

Lubrication (2-cycle engine only)

  • Out of oil
  • Improper oil or fuel/oil ratio

Low or Loss of Compression

  • Cracked or broken piston rings
  • Damaged head gasket or cracked cylinder/cylinder head
  • Valve sticking or broken push rod
  • Pre-ignition damage
  • Detonation damage

Engine Overload

  • Engine operated in conditions above those for which designed
  • Valve (4-cycle engine only)
  • Improper adjustment in lash
  • Improper seating due to burned, bent or stuck valve
  • Broken valve spring
  • Valve or valve seal leaking

Exhaust Restriction

  • Damaged exhaust system
  • Clogged or restricted baffles or exhaust port

Knocks or Pings

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

Oil or Fuel/Oil Ratio (2-cycle engine only)

  • Improper mix
  • Separation of oil and fuel
  • Main bearing seal leakage

Cooling (2-cycle engine only)

  • Restricted in cooling system or fins
  • Lack of coolant
  • Cooling fin damage
  • Damaged head gasket

Lubrication

  • Out of oil
  • Improper oil or fuel/oil ratio

Engine Overload

  • Engine operated in conditions above those for which designed

Operates Erratically

Improper Fuel/Air Ratio

  • Inspect choke operation
  • Improper carburetor settings/adjustments
  • Air leak in intake system
  • Damaged head gasket or cracked cylinder/cylinder head
  • Leaking crankcase seals
  • Engine flooded
  • Dirty fuel filter
  • Defective fuel pump

Faulty Governor

  • Governor assembly malfunctioning
  • Linkage out of adjustment, worn or disconnected
  • Governor spring sensitivity too great
  • Linkage binding

Lubrication

  • Out of oil
  • Improper oil or fuel/oil ratio

Carbon Buildup (2-cycle engine only)

  • Normal accumulation
  • Excessive rich mixture, fuel/oil ratio
  • Incorrect oil type
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