Fuel Supply, Carburetion, Fuel Injection, Exhaust and Emission Control
What the exam tests
How petrol is metered and mixed with air, how carburettor circuits and electronic fuel injection differ, and how exhaust and emission-control devices meet BS-VI limits.
Carburettor circuits
- Float chamber keeps a constant head of fuel; float height is set by bending the tang, never by force.
- Pilot (idle) circuit and mixture screw control up to about 1/4 throttle.
- Jet needle and needle jet (with the slide) control 1/4 to 3/4 throttle; the clip position raises or lowers the needle.
- Main jet controls 3/4 to full throttle; a larger number is richer.
- Choke or enricher, and an accelerator pump on some models, cover cold starting and sudden opening. A constant-velocity carburettor uses a diaphragm slide so airflow, not the twistgrip, lifts the slide.
Fuel injection and emissions
Stoichiometric mixture is about 14.7:1. An EFI system has a pump, filter, injector, throttle body with TPS, MAP or IAT sensor, an oxygen sensor and the ECU, which corrects fuelling in closed loop. Emission control: three-way catalytic converter, EVAP canister for fuel vapour, PAIR or secondary air for oxidation, and OBD-II monitoring on BS-VI machines. Exhaust leaks upset the oxygen sensor and force rich or lean corrections.
Worked example
A motorcycle runs rich at half throttle: the plug is sooty, exhaust smokes and economy is poor, while idle and full throttle are acceptable. Reasoning: the jet needle and needle jet meter the mid-range. The clip sits at the bottom groove, holding the needle high. Move the clip up one or two grooves to lower the needle, then re-read the plug colour after a road test.
Traps
Turning a sealed mixture screw, adjusting float height by bending the float arm, diagnosing a lean mixture without checking for intake leaks, and fitting a free-flow exhaust without ECU recalibration.