Sri Lanka NVQ L3 Motorcycle Mechanic flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Stoichiometric air-fuel ratio

The mass ratio of air to fuel that gives complete combustion, about 14.7:1 for petrol. Richer mixtures give more power and cooler combustion, leaner mixtures give economy but higher temperature.

Compression ratio formula

Compression ratio equals swept volume plus clearance volume, divided by clearance volume. A 150 cc cylinder with 15 cc clearance volume gives 165 divided by 15, or 11:1.

Four-stroke cycle order

Intake, compression, power and exhaust, completed in two crankshaft revolutions. Each valve opens once per cycle, and the spark occurs near the end of compression, before top dead centre.

Valve clearance measurement

Measured with a feeler gauge between cam and follower or rocker, engine cold at top dead centre on the compression stroke. Correct clearance is the largest gauge that enters with slight drag.

Piston ring end gap

Checked by squaring the ring in the cylinder bore with a piston crown, then measuring the gap with a feeler gauge. Too small a gap causes ring breakage when hot; too large causes blow-by.

Micrometer least count

A metric micrometer with a 0.5 mm main scale and a 50-division thimble reads to 0.01 mm. Reading equals main scale plus thimble division multiplied by 0.01 mm.

Vernier caliper least count

Least count equals one main-scale division divided by the number of vernier divisions. A 1 mm main scale with 20 vernier divisions gives a least count of 0.05 mm.

Torque formula

Torque equals force multiplied by perpendicular distance from the pivot, T = F x d. A wrench reading is valid only when force is applied at 90 degrees at the marked handle point.

Plastigauge bearing clearance

A plastic strip placed across the journal and crushed by the torqued cap, without rotating the crank. The flattened width is compared with a scale to give oil clearance.

Cylinder taper and out-of-round

Measured with a bore gauge at three heights and two planes, parallel and perpendicular to the crankshaft. Wear beyond about 0.05 mm or beyond spec requires reboring or replacement.

Carburettor circuit ranges

Pilot circuit covers idle to about one quarter throttle, needle and slide cover one quarter to three quarters, and the main jet covers three quarters to full throttle.

Float level effect

Fuel level in the float bowl affects every carburettor circuit. A high level causes richness and flooding; a low level causes lean running, hesitation and poor starting.

Ignition timing BTDC

Spark timing stated in degrees before top dead centre. Over-advanced timing causes detonation and pinging; over-retarded timing causes overheating, poor power and exhaust popping.

Spark plug colour reading

Light tan indicates normal combustion. Black soot indicates a rich mixture or weak ignition. White, blistered or melted electrodes indicate a lean mixture or overheating.

Oil grade 10W-40 meaning

The 10W is the cold (winter) viscosity grade and 40 is the hot viscosity grade. Motorcycle wet clutches require JASO MA or MA2 rated oil to prevent slipping.

Radiator cap pressure function

The cap raises coolant boiling point above 100 degrees Celsius, roughly 120 degrees at 0.7 bar. Pressure must be released only when the system is cold.

Coolant mixture ratio

Typically 50 percent water and 50 percent ethylene glycol, giving freeze and boil protection plus corrosion inhibitors. Concentration is checked with a hydrometer or refractometer.

Clutch lever free play

The small gap, typically 10 to 20 mm at the lever, that ensures the clutch fully engages and the release bearing is unloaded when the lever is released.

Chain slack measurement

Measured at the midpoint of the lower chain run, typically 20 to 30 mm. Too tight overloads output shaft bearings; too loose risks derailment and case damage.

Overall gear ratio

Primary ratio multiplied by gearbox ratio multiplied by final drive ratio. A numerically lower overall ratio gives higher speed; a numerically higher ratio gives better acceleration.

Pascal's law in brakes

Pressure applied to a confined fluid is transmitted equally in all directions. A caliper piston with larger area than the master cylinder piston multiplies clamping force.

Brake fluid hygroscopy

DOT 3 and DOT 4 glycol fluids absorb moisture from air, lowering boiling point and causing spongy brakes. DOT 5 silicone fluid must never be mixed with glycol fluid.

Tyre size 120/70 R17

120 mm section width, 70 percent aspect ratio (sidewall height 84 mm), radial construction, 17 inch rim. Load index and speed symbol must meet the motorcycle requirement.

Rake and trail

Rake is the steering axis angle from vertical; trail is the horizontal distance from the steering axis to the tyre contact patch. More trail gives stability, less gives quicker steering.

Ohm's law and power

V = I x R, and P = V x I. A 12 volt, 60 watt headlamp draws 5 amperes and has about 2.4 ohms resistance.

Battery open-circuit voltage

A fully charged 12 V lead-acid battery reads about 12.6 to 12.8 V. About 12.0 V is roughly half charged, and below 11.8 V is discharged and needs charging.

Charging voltage check

At 2,000 to 3,000 rpm the battery should read about 13.5 to 14.5 V. Lower suggests a failed regulator, stator or wiring; higher suggests a failed regulator.

Voltage drop test

Measures resistance in a live circuit by loading it and reading voltage across the cable or earth. A drop above about 0.2 V indicates a poor connection.

Compression test procedure

Warm engine, throttle held open, gauge fitted in the plug hole, crank until the reading peaks. Compare with spec and between cylinders; large variance indicates ring or valve faults.

Cylinder leak-down test

Air is fed into the cylinder at top dead centre and leakage is read as a percentage. Air from the exhaust means exhaust valve leak, from the intake intake valve leak, from the breather rings.

Hydrogen battery hazard

Lead-acid charging releases hydrogen, which is explosive. Charge in a ventilated area, keep flames and sparks away, and add acid to water, never water to acid.

Fire extinguisher class rule

Petrol and flammable liquid fires need foam, dry powder or CO2, never water. Electrical fires need CO2 or dry powder. Metal fires need a class D extinguisher.