Fuel, Ignition, Lubrication and Cooling Systems
What this domain tests
This section spans carburettor and fuel-injection basics, air-fuel ratio, ignition coils, CDI/TCI, spark-plug heat range and gap, ignition timing, wet-sump lubrication, oil pumps and filters, and air or liquid cooling. Expect cause-and-effect questions linking mixture and ignition to overheating, fouling and knock.
Core rules
- Stoichiometric petrol-air ratio is about 14.7:1 by mass. Lean mixtures burn hotter and can cause detonation; rich mixtures cool the charge but foul plugs and waste fuel.
- A carburettor uses a venturi pressure drop to draw fuel through the pilot, needle and main circuits. Fuel injection meters fuel electronically using sensors and an ECU.
- Spark-plug gap is typically 0.7–0.9 mm on many small motorcycle engines. A plug that is too hot a heat range can pre-ignite; too cold fouls.
- Ignition timing is set with a timing mark or ECU map. Vacuum or electronic advance changes timing with speed and load.
- Wet-sump lubrication uses a pump, oil filter and galleries. Oil grade must match ambient temperature and manufacturer specification.
- Air-cooled engines rely on fins and airflow; liquid systems use a pump, thermostat, radiator and pressure cap. Never open a hot radiator cap.
Worked example
An engine consumes 10 g of petrol. At 14.7:1 it needs 10 × 14.7 = 147 g of air. If the intake supplies only 120 g, the mixture is rich, not lean, because fuel exceeds the air available. The correction is to restore airflow or reduce fuel, not to advance ignition timing.
Traps that cost marks
Assuming a lean mixture is safe because it gives better economy; fitting a hotter plug to cure fouling; over-gapping a plug and damaging the coil; adding the wrong coolant; ignoring an air lock after refilling.
Key points
- Stoichiometric petrol-air ratio is about 14.7:1 by mass.
- Lean mixtures raise combustion temperature.
- Spark-plug heat range must match engine demand.
- Oil grade depends on ambient temperature and specification.
- Liquid cooling needs thermostat, pressure cap and air-free system.