Lubrication and cooling systems
This domain tests oil flow, oil quality and heat management on air- and liquid-cooled motorcycles. Questions cover pump and filter function, oil grades, coolant circuits and overheating diagnosis.
Lubrication rules
- A wet-sump system stores oil in the crankcase; a dry-sump system uses a separate tank. The pump draws through a strainer and feeds the filter and galleries.
- Use the grade in the manual. JASO MA or MA2 is specified for wet clutches; API resource-conserving oils with friction modifiers can make the clutch slip.
- Check oil level with the bike upright, engine warm, after the specified wait, without screwing in the dipstick unless stated.
- An oil pressure warning at idle can mean low level, a worn pump, a blocked pickup or a stuck relief valve. Stop the engine.
Cooling rules
- Air-cooled engines rely on fins and airflow; bent fins, mud and idling in traffic cause overheating.
- Liquid systems have a radiator, pressure cap, thermostat, water pump and fan. The cap raises boiling point; never open it hot.
- Use the correct coolant type; mixing silicate and organic coolants can gel. Bleed air from the highest point after refilling.
- A thermostat stuck closed overheats quickly; stuck open causes slow warm-up and poor performance.
Worked example
A liquid-cooled 150 cc overheats in slow traffic but is normal at speed. The coolant is full and the fan does not run when the gauge is hot. Reasoning: airflow through the radiator is fine at speed, so the fault is fan operation. Check the fan fuse, then bridge the thermo-switch connector with the ignition on; if the fan runs, replace the switch.
Traps
Overfilling oil; using car oil in a wet clutch; opening the radiator cap hot; adding water only; ignoring a weeping water-pump seal.