Fuel, ignition, intake and exhaust systems
This domain covers mixture formation, ignition timing, air intake and exhaust flow. It tests carburettor and EFI fundamentals, spark plug condition, sensor function and exhaust restriction.
Core rules
- Stoichiometric petrol mixture is about 14.7:1 by mass. Rich mixtures cool combustion but soot the plug; lean mixtures raise temperature and risk detonation.
- Carburettor circuits: float level, pilot/idle, needle jet, main jet, choke or enricher. Float height is set with the needle valve just closed, not compressed.
- EFI uses sensors: TPS for throttle position, MAP or MAF for load, ECT and IAT for temperature, O2 for closed-loop correction. A vacuum leak leans the mixture at idle.
- Spark plug heat range: a hot plug for low-speed running, a cold plug for sustained high load. Gap and torque matter; overtightening cracks the insulator.
- Ignition timing is set with a timing light or stored in the ECU. Too much advance causes knock; too little causes overheating and poor power.
- Exhaust back pressure matters; a collapsed baffle or blocked catalytic converter costs power.
Worked example
A carburetted 125 cc runs rich at idle with black smoke and a sooty plug. The air filter is clean and the choke is off. Reasoning: fuel level is too high or the pilot circuit is oversized. Check float height first; if fuel drips from the overflow, the needle valve is worn or the float is sunk. Set float height to specification, adjust the idle mixture screw for the highest steady idle, then reset base idle.
Traps
Adjusting the mixture screw to hide a dirty pilot jet; fitting a colder plug to cure overheating; assuming EFI needs no maintenance; bending the float tab without measuring; leaving the enricher partly on.