4 STROKE ENGINE
Many have requested a diagram of a four stroke engine like the GY6. This is a good representation except that the GY6 has rocker arms actuated by a camshaft that open the valves. Just imagine that it's horizontal rather than vertical, the exhaust is on the left which faces down so your scooter's front would be to the left. Top dead center (TDC) is when both valves are closed and you see the explosion of the gases. A lot of scoot experts feel you should set your valve clearance at this point. However, if you look carefully you'll see that when one valve is completely open, the other valve is always closed no matter what the phase of the engine; therefore the closed valve, which has no tension on it, may be set. This is an alternative method for setting the valves as opposed to the TDC method. Simply remove the plug, use the kick start pedal to rotate a cam lobe until it's completely against it's rocker arm, set the other valve; repeat with the other valve. Here is the animation, stroke #1 is intake, #2 is compression and ignition, #3 is power, and #4 is exhaust.
The next is a very cool cutaway of a GY6 showing the piston, crank, intake valve, intake port, starter, connecting rod, etc.:
Oil Change (video)
Valve adjustment; this guys procedure is good but most would disagree with his settings. About .004" intake and .006" exhaust on 150cc is common, although there is lots of discussion on this; and about .002" on 50cc, again, much discussion. Valve Adjustment (video)