A periodic timing adjustment may be required on some vehicles; some cars can lose their timing and some maintenance requirements, such as timing belt replacements, can affect the timing.
What Is Timing?
On a combustion vehicle, the timing refers to the exact interval needed between the turning of the crankshaft at the center of the engine block and the turning of the camshafts and distributor. This controls the fuel and air injection as well as the spark. As the crankshaft turns, a timing belt or chain will run from that to the camshaft pulleys and turn the camshafts in time with the crank. The turning camshafts have egg-shaped knobs on them which push and release valves in the cylinder heads, controlling the release of air and fuel. At the same time, gearing on the camshaft turns the distributor causing the cap and rotor to connect with the proper wires to fire the spark plugs in time with the fuel-air injections controlled by the camshafts.
Having perfect timing means your engine is in tune and running at maximum performance and efficiency. If the timing is just slightly off, it can cause the engine to run roughly, misfire, or even not run at all.
How Is the Adjustment Made?
Top dead center
Most of the time, a timing adjustment is accomplished by setting the engine at what's called 'top dead center' (TDC). Marks on the camshaft pulleys and crankshaft pulley will show where this is. When all are lined up on those marks, the engine is at TDC.
Align the belt
At this point, the timing belt should be aligned on the pulleys so that when one turns, the rest turn in time with it.
Adjust the distributor
After the timing belt adjustment is accomplished, the distributor is adjusted until timing is exactly right so that cylinders are firing perfectly.
This is usually accomplished using a special tool called a timing light. It clamps over the first cylinder's spark plug wire and, as power is sent through the wire, triggers a light on the head on a wand. This light, when pointed at chalk marks to indicate TDC, will show the mechanic which way to adjust the distributor to get the timing correct.
Although timing adjustment is an easy process to go through, it is not one that some beginner DIY mechanics should undertake as it requires a fair amount of knowledge of how the combustion engine operates. Experienced home mechanics will have no trouble.