The four strokes of a conventionally aspirated petrol/gasoline engine are:-
Each upstroke/downstroke combination corresponds to a single revolution of the crankshaft. Thus, in a 4-stroke engine, each individual cylinder fires only once for every two revolutions. However, with four cylinders, that means a total of four ignition discharges for every two revolutions, i.e. two discharges per revolution.
An induction-operated tachometer, if it was designed for use on a 4-cylinder 4-stroke engine, counts the number of discharges per minute by monitoring part of the ignition system, typically the ignition coil. It then does the arithmetic and displays half this value as the number of Revolutions Per Minute (RPM).
This engine has no valves. The fuel-air mixture enters the combustion chamber via an inlet port in the side of the chamber. This port is uncovered by the piston only towards the bottom end of the stroke. Likewise, spent gases leave the chamber via an exhaust port situated on the opposite side of the chamber to the inlet port. Again, this port is uncovered only towards the bottom end of the stroke.
The two strokes are:-
The obvious difference between the 2-stroke and 4-stroke engines is that every cylinder in the 2-stroke engine fires once per revolution. So, in a single-cylinder 2-stroke engine, counting the number of ignition discharges is the same as counting the number of RPM.
The arithmetic is not too difficult. If the gauge "thinks" it is monitoring a 4-cylinder 4-stroke engine it counts the number of discharges per minute, divides by two, and displays the answer as the RPM. If, in fact, it is monitoring a single-cylinder 2-stroke engine, then it ought not to have divided by two. So, when a tachometer set to "4-cylinder 4-stroke" is fitted to a single-cylinder 2-stroke engine, the reading displayed is half the true RPM, e.g. 600 RPM displayed on the gauge corresponds to 1200 RPM on the engine.