I have attached some photos of the inside of an engine. Yours will be different as these are Briggs engines, but you can get the idea. The two dots/dashes are the timing marks. The first shows the crankshaft & cam shaft gears aligned properly, the second shows them out of alignment (different engine). The second engine still ran but just barely.
The cam gear (bigger gear) is driven by the crankshaft gear (the smaller one). As the piston rises and falls, the crankshaft rotates and turns the cam shaft which raises and lowers the valves. There are 4 stroke in your engine: suck, squeeze, bang, blow, also known as intake, compression, ignition, exhaust. It is not enough that the piston and valves rise and fall, they must do so at specific times. That timing is controlled by the alignment of those gears. If they are not aligned properly, then the valves may LOOK like they are opening and closing at the correct time, but they ARE NOT. The intake valve MUST be closing on the compression stroke and fully closed at spark, or the engine will blow fuel / air mixture & / or fire out the carb.
The cam gear is twice as big as the crankshaft gear. The cam gear goes around once for every 2 rotations of the crankshaft gear. The cam shaft drives the points, which means they open once for every two revolutions of the crankshaft (4 strokes). It must happen just before TDC on the compression stroke. If your gears are out of align, your spark will arrive when the piston is somewhere other than short of TDC of the compression stroke. It may happen 17 degrees early, or 17 degrees late (720 degrees / 42 teeth). You cannot fix that with the points gap and your engine may run, but it will be firing while the intake or exhaust valve is still open. It can't be 180 degrees out, or it would firing at TDC on the exhaust stroke, and it would not convert fuel to noise.
To further complicate matters, some engines have a compression release mechanism that cracks opens one of the valves on the compression stroke at low RPMs in order to facilitate starting by reducing the compression and making it easier to rotate the engine. If that fails, and stays on, then the corresponding valve will bump open of the compression stroke, possibly causing the engine to fire out the carb or exhaust.
All of that means you have to open the engine to see what is not where it belongs. Hope that helps.
Edited by New.Canadian.DB.Owner, June 28, 2016 - 09:00 PM.