difference between 2-stroke and 4-stroke, in the most basic terms, is that a 2-stroke engine has 2 pisont strokes to every cycle (up then down). whereas a 4-stroke engine has 4 strokes in every cycle (up down up down) so the fuel is only combusted 1 in 2 times the piston moves down, the other time it is still moving with the momentum fromt he previous combustion.