Fuel, Ignition, Lubrication and Cooling Systems
Fuel system
Carburettors meter fuel by venturi vacuum through pilot, needle/jet and main circuits; fuel injection uses sensors (TPS, MAP/MAF, oxygen, coolant and air temperature) and an ECU to set pulse width. Know the effects of a lean mixture (hard starting, surging, overheating, white plug) versus rich (black smoke, fouled plug, poor economy). Fuel pressure, filter restriction and tank venting cause many faults. Ethanol-blended fuel absorbs water and degrades in storage.
Ignition
Battery-coil (inductive) and capacitor-discharge (CDI) systems. Key terms: dwell/charge time, coil primary resistance, pickup/trigger gap, spark plug heat range, and timing advance with rpm. Too hot a plug (high number in NGK, low number in Bosch/Champion conventions) can pre-ignite; too cold fouls. Check for spark with a proper tester, not by holding the lead.
Lubrication
Oil performs lubrication, cooling, cleaning, sealing and corrosion protection. Know viscosity grades (SAE 10W-40: the W number is cold-flow behaviour, the second number is hot viscosity), API/JASO ratings, wet-clutch compatibility (friction modifiers can cause clutch slip), oil pump type, filter bypass, and the meaning of oil pressure warning at idle. Sludge indicates short trips or missed changes.
Cooling
Air cooling relies on fin condition and airflow. Liquid systems: radiator, cap pressure rating, thermostat, water pump, hoses, fan switch. Coolant is a mix of ethylene glycol and water plus corrosion inhibitors; a 50/50 mix is typical. Never open a hot radiator cap; bleed the system and check the fan operates.
Traps
Confusing plug heat-range direction between manufacturers; assuming a no-start is ignition when it is fuel; ignoring a pressure cap that will not hold pressure (overheating at load).
Rule of thumb
Diagnose by symptom pattern, not by part. A plug reading after a plug chop is the fastest mixture indicator.