* Timing Chain's Role: The timing chain synchronizes the rotation of the crankshaft (which drives the pistons) and the camshaft (which operates the valves). If the chain is stretched, broken, or otherwise out of time, the valves and pistons won't be operating in the proper sequence.
* Result of Mistiming: If the valves are open when the piston is at top dead center (TDC) on the compression stroke, the piston will hit the valves, causing significant damage. This is called "valve interference." Even if there's no immediate catastrophic damage, the severely out-of-sync timing will prevent the engine from firing correctly and starting.
* Spark and Fuel Aren't Enough: While you're getting spark and fuel, the engine needs the precise timing of those events relative to the piston position to create combustion. A faulty timing chain throws this timing off completely.
In short, while spark and fuel are necessary for starting, proper valve timing is equally crucial. A problematic timing chain disrupts this timing, leading to a no-start condition even with spark and fuel delivery.
