Explain the function of ignition points, mechanical advance units, and spark plugs in an engine, and how to set and check ignition timing.

Here are the answers to your questions:

1. Why are ignition points fitted? Ignition points are fitted in older ignition systems to interrupt the primary circuit of the ignition coil, which induces a high voltage in the secondary circuit, causing the spark plugs to fire.

2. Why does the engine need a mechanical advance unit? The engine needs a mechanical advance unit to advance the ignition timing as engine RPM increases. This ensures that the air-fuel mixture ignites earlier at higher speeds, allowing sufficient time for complete combustion and optimal power output.

3. When setting the timing, do we set it before TDC or after TDC? When setting the timing, it is almost always set before TDC (Top Dead Center). This allows the combustion process to begin and build pressure, reaching its peak just after the piston passes TDC for maximum efficiency.

4. How do we check the ignition timing when the engine is running? We check the ignition timing when the engine is running using a timing light. The timing light flashes in sync with the spark plug firing, illuminating timing marks on the crankshaft pulley or flywheel to verify alignment with a reference mark.

5. Why is a spark plug fitted to an engine? A spark plug is fitted to an engine to ignite the compressed air-fuel mixture in the combustion chamber. It creates an electrical spark that initiates the combustion process, driving the piston.

6. If the air cleaner was restricted, how would the spark plug look like? If the air cleaner was restricted, the engine would run rich (too much fuel, not enough air). The spark plug would likely appear black and sooty due to incomplete combustion and carbon deposits.

7. The engine is burning oil, what would the spark plug look like? If the engine is burning oil, the spark plug would typically look oily, wet, and black, often with a glazed or carbonized appearance, due to oil fouling.

8. How often should the spark plug be replaced? Spark plug replacement frequency varies significantly based on the vehicle manufacturer's recommendations and the type of spark plug (e.g., copper, platinum, iridium). Intervals can range from every 30,000 miles to over 100,000 miles.