Malfunction and cause	Remedy
2. Engine starts but idles irregularly
Carburetor settings too lean or too rich	Adjust carburetor settings
Valve clearances too small	Adjust valve clearances
Valve blow-by (mixture leakage)	Have valves reground
3. When warm, engine idles irregularly, sooty exhaust
Fuel feed valve leaking, idle mixture setting too rich	Clean or renew valve, adjust idle settings
4. Engine runs erratically, misfires at intervals
Spark plug electrode gap too large	Adjust gaps
Spark plugs oiled or sooted up	Clean or renew spark plugs
Ignition leads wet or defective	Dry or renew leads
Short-circuit at spark plug cap (visible as sooty burn marks)	Clean or renew plug caps
Defective ignition system	Renew defective components
Carburetor jets blocked	Clean jets
Fuel line blocked	Clean fuel line
Moisture condensate in float chamber	Dry out and clean float chamber
5. Engine overheats, runs on when ignition is switched off
Fuel/air mixture too lean	Check carburetor, adjust settings.
	Check that correct jets are fitted
Ignition timing has slipped	Check ignition, retime
Blocked engine cooling fins	Clean cooling fins
Thermal value of spark plugs too low	Use the specified spark plug rating