Fuel, Ignition, and Emission Control Systems
This domain covers carburation, fuel injection, spark delivery and the hardware that keeps exhaust emissions in check.
Core rules
- Carburettor circuits are throttle-position dependent: pilot circuit (0–1/8), slide cutaway and needle (1/8–3/4), main jet (3/4–full). The idle mixture screw affects only the first 1/8.
- Float height sets fuel level; a high level richens every circuit, a low level leans them all.
- EFI inputs you must know: crank position (CKP) for timing, MAP/TPS for load, ECT/IAT for correction, O2 for closed-loop trim. A biased or lazy O2 sensor causes rich/lean drift.
- Fuel pressure is a specification - measure at the rail with a gauge before condemning injectors.
- Spark plugs read mixture and heat range: black soot = rich, blistered white = lean or overheating, tan = correct.
- Emissions hardware: catalytic converter (needs correct mixture to function), evaporative canister and purge valve, PAIR/air injection to oxidise CO and HC in the exhaust.
Worked example
A bike surges and lean-hunts at a steady 1/4 throttle but pulls cleanly at wide open. Reasoning: wide-open is main-jet territory and is fine, so fuel supply and ignition are adequate at high demand. The fault sits in the transition zone - needle clip too low (needle too lean), a worn needle jet, or a leaking slide diaphragm. Check the diaphragm first because it is cheap and common, then raise the needle one clip and re-test the same throttle opening.
Traps that cost marks
- Blaming jetting for a lean condition caused by an intake air leak at the boot.
- Adjusting the idle mixture screw to cure a mid-range problem.
- Fitting a free-flowing can or filter without addressing fuel trim, then chasing a lean code.