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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