Two strokes have many virtues, including simplicity – because they have no valvetrain – and low cost (because they have fewer moving parts) and high output for their displacement, relative to a four stroke engine of the same displacement, because two stroke engines make power every time a piston ascends to top dead center within the cylinder. A four stroke engine has four strokes – intake, compression, combustion, exhaust – but only one of those strokes (combustion) results in power.

Two strokes also have some strikes against them. The main one being they are very difficult to make compliant – with emissions standards – because for one thing they burn oil on purpose (it is mixed with the gas, to provide engine lubrication) and for another because the nature of the design allows for contamination of the intake charge with exhaust gasses. which is a function of using ports that are covered and uncovered by the pistons as they go up and down in the cylinders. Four strokes also have ports but they are opened and closed by intake and exhaust valves that seal when closed.

There is also the related problem of piston ring wear caused by the piston going up and down in cylinders that have holes (those ports) in their sides. In a four stroke engine, the piston is surrounded completely by the cylinder wall, which compresses the piston and oil control rings evenly. In a two stroke engine, the open port on the side of the cylinder creates a spot where the rings are not compressed evenly, leading to faster wear and more blow-by (higher emissions). And that is why the only vehicles with two stroke engines that can still be legally sold are for use off-road only.