Engine Troubleshooting
No Start/No Spark
• ignition not on
• kill switch off or dirty
• low battery
• fuse
• loose or corroded battery terminals
• old, dirty, worn spark plugs
• plugs wet
• wrong plug gap
• plug caps and resistors
• points pitted, dirty, wrong gap, broken cable
• point earthing to plate, terminal insulators broken
• coils
• condensors
Spark but No Start
• no fuel
• fuel delivery - filters, petcocks
• valve timing
• ignition timing
• compression - top end, rings, head gasket, holed piston
• carburetor settings
• float needle, leaky float, float settings
• pilot circuit blocked
• tank cap vent
Spark (one side only)
• plugs - worn, fouled, defective, dirty, wrong gap
• plug leads, cap and resistor
• points gap, pitting, dirty, earthing, broken wire
• condensor
• coil
Hard To Start
• battery flat
• plugs - worn, dirty, gapping, too cold, kaputt
• points - pitted, dirty
• leads cracked, old, dirty
• coils, condensors
• broken or intermittent earth or connections at points, coil
• cam, valve, points timing out
• valve adjustment
• valve seats
• low compression
Starts but Won't Run
• fuel delivery
• plugs worn or too cold
• tappet setting
• ignition timing
• yellow to safety relay
Rough Idle
• plugs – worn, dirty, gapped too wide, too cold
• spark weak - coils, leads
• points gap wrong, pitted
• ignition timing
• atu
• valves not sealing - worn valves, seats, guides
• tappet adjustment
• leaking manifolds both front and rear, butterfly shaft seals
• float height
• petcocks
• water in bowls
• idle adjustment incorrect
• pilot circuits blocked...take note if running cheap pod filters, they block the pilot air jets
Top-End Noise
• loose tappets
• cam chain loose or worn
• cam/crank-shaft sprocket worn
• ping-spark knock
• worn gudgeon pin and/or small end
• piston knock - worn cylinder
Bottom-End Noise
• big end bearing knock - esp noticeable at idle
• main bearings
Transmission Noise
• gears worn, pitted
• main and/or counter -shaft splines worn
• primary gear worn
• noisy clutch
Runs/Misfires On Acceleration
• battery low
• tank cap vent
• water in bowls
• blocked main jet
• diaphragm
• air leaks
• intermittent or loose connection in ignition wires
• ignition timing
• carb settings incorrect
• coils, condensors
Surges/Runs Unevenly
• fuel delivery
• too lean
• air leaks - manifolds front and rear, butterfly shaft seals, vacuum barbs, diaphragm, choke seal, bowl connectors
• valve adjustment
Misfires, Breaks Up When Running
• low battery
• loose battery cable
• loose or intermittent contact in ignition circuit
Popping/Misfires On Deceleration
• too lean
• exhaust leaks
Backfires/Kickback
• too advanced
• atu not working
Overheating
• too lean
• too advanced
• too retarded
• oil low or thin
• oil pump defect, blocked galleries
• carbon choking
• plug too hot
• atu not working properly
Poor Operation - Low Speed
• tank cap vent
• battery weak
• weak spark
• ign timing out
• plugs too cold or gap too big
• bad points contact
• valve adjustment
• valve timing incorrect
• valve seat leak
• worn valve guides
• pilot circuit dirty...take note if running cheap pod filters, they block the pilot air jet
• air mix screw not set right
• float level wrong
• air leak
• carb equaliser tube leaking or loose
• carburetor not level
• petcocks blocked
• dirty gas
Poor Operation - High Speed
• ign timing retarded
• atu
• plugs too cold, gap too small
• points spring weak
• valve timing
• weak valve springs
• broken rings
• coils, condensors
• torn diaphragm
• butterfly valve not opening, not synchronised
• blocked or wrong main jet
• wrong needle setting
• choke on
• carbs not level
• float level too low
• fuel line, filter, petcock partly blocked
• air filter dirty
• dirty gas
Loss Of Power
• tank cap vent
• exhaust pipe loose or broken
• carbon clogging - engine, mufler
• air filter dirty
• ign timing
• plug gap
• valve adjustment, timing
• valve seating
• weak valve springs
• worn cylinder or rings
• blown head gasket
• float levels too low
Smoking - Blue, Oil Consumption
• too much oil
• rings - bore worn
• marked bore
• valve guides, seals
• crankcase breather blocked
Smoking - Black
• too rich
• engine is carbon clogged
Seized Piston(s)
• oil level too low
• oil pump not working
• overheating - too lean, too advanced, stuck valves, tappet clearance too small
Worn Piston Rings/Bore
• low. thin or contaminated oil
• oil pump defect
• no warm-up
• air filter damaged
Valves - Bent, Broken, Burnt
• weak springs
• overrevving
• hitting piston - wrong valve timing
• tight clearances
• too retarded
Connecting Rod Bearings
• oil low, wrong viscosity
• oil-contaminated
• overrevving
• too advanced, misfiring at high speed
Crankshaft Bearings
• oil low, wrong viscosity
• oil-contaminated
• no warm-up
• overrevving
Camshaft - Worn Lobes or Bearings
• oil low, too thin or contaminated
• no warm-up
• oil pump not working, oil galleries blocked
• tight tappet clearances