FUNCTIONING OF ENGINE

VALVE TIMING
The valves are opened by the CAMSHAFT. The camshaft is in essence just a bar of metal with projections (CAMS) sticking out along its length. As the camshaft rotates these projections move the valves to an open position.
Valve closure is accomplished by the use of springs. The position of the projections on the Camshaft is set by the manufacturer but the controlling of the rotation is governed by its assembly into the engine. The camshaft rotates at half of the speed of the crankshaft and is driven from the crankshaft via gears, chains or a toothed rubber belt.
On all engines you will find "timing" marks to help you re-align the camshaft on re-assembly (i.e. The installation of a new toothed belt).
If there are no timing marks to be found, then, an approximate timing can be afforded be positioning the first piston at the top (TDC Top Dead Centre) and setting both valves to overlap (exhaust nearly closed and the intake nearly open).
IGNITION TIMING
The spark itself is produced in a transformer coil (high-tension coil). This coil converts the 12 volts from the battery into many thousands of volts and is created at just the right moment by the opening of a "contact breaker" or electronically by the use of magnets and "Hall effect transistors" (electronic ignition).
As you can imagine the spark generation also has to be at the right time. Too late and the ignition is "retarded" causing overheating due to the very small power output. Too early and the engine will probably backfire, (through the carburettor), run very roughly and sound terrible.
This is all controlled by the positioning of the "Distributor". The drive to most contact breaker shafts is also at half of the crankshaft speed. The distributor is employed for "distributing" the high voltage to the correct cylinder's spark plug. You will see a plastic cap with many leads ensuing. One lead you can trace back to the high-tension coil. The total number of the other leads matches that of the number of cylinders in the engine. The high voltage enters the centre of the distributor cap and is fed via the "rotor arm" to one of the other leads that go to the spark plugs in the "cylinder head". These are set to the "Firing Order" of the engine, which is usually embossed somewhere on the intake manifold. The firing order on a four-cylinder engine might be for example --1, 3, 4, 2 --.
So with this information you can replace these leads if they have become inadvertently mixed up. All you need to ascertain is the direction of rotation of the "Rotor Arm" within the distributor by turning the engine over (with the use of the ignition key or by selecting a low gear and pushing the vehicle forward). Then you replace the leads in the order given (i.e. 1, 3, 4, 2) following the direction of rotation you ascertained from above. The number 1 cylinder is usually nearest to the radiator (front of the car).
The first lead you connect is the most critical for getting the whole sequence in the correct order. The rotor arm will be pointing at one of the leads and this lead must go to the cylinder that is under compression at that time. (This is ascertained by observing which cylinder has not got a valve open at that time).
THE CARBURETTOR
The carburettor controls the amount of fuel in relation to the amount of air that enters the engine. This is carried out by the deployment of a butterfly valve, which is linked in turn to the throttle pedal in the vehicle.
The ratio of Fuel to Air is determined by the size of the holes in the "JETS" within the carburettor and is set by the engine designers. But other factors do come into play. The "choke" for example restricts the airflow in the carburettor and thus more fuel, in turn, is administered to the engine (necessary to compensate for the condensation of fuel inside a cold engine). Also a blocked air filter will have the same effect of restricting the air and forcing more fuel into the engine causing fuel wastage through inefficiency and poor running of the engine. Blocked "Jets" on the other hand allow less fuel to enter the engine and weaken the mixture giving rise to overheating and also poor running.