We’re all familiar with big capacity four-stroke singles, because they’re the most interesting. But a third of today’s single-cylinder bikes are much smaller-capacity two-strokes; usually mopeds and learner bikes, with a smattering of mad-as-you-like motocrossers.
A two-stroke is much simpler than a four-stroke. There are no camshaft-operated valves, just (in general) ports in the cylinder barrel that let fuel and air in or out when they’re uncovered by the piston. Sometimes the mixture comes in via the crankcase, controlled by a rotating disc with a cutaway in it that’s uncovered on every rotation. In a four-stroke, the mixture goes in and out via poppet valves controlled by the camshaft.
The big difference, though, is that every time a two-stroke’s piston reaches the top of its cylinder, the spark plug ignites the mixture – so there’s a firing stroke every 360 degrees. In a four-stroke the plug only ignites the mixture every other time the piston reaches the top of the cylinder – every 720 degrees.
That makes a two-stroke single smoother than a four-stroke single and, with the lack of moving components, allows it to rev higher too. But the two-stroke principle doesn’t work well as cylinder capacities go up, which is why you’ll rarely find a single-cylinder two-stroke over 125cc.