The "theoretical" fuel-air ratio is the same for any gasoline engine. Two-stroke engines seem to be set to run richer, perhaps because cooling the piston on a two-stroke is more difficult. A two-stroke piston has a power stroke every revolution, and it can't have an oil jet on its bottom for cooling. Running rich means the extra unburned fuel, by evaporation, carries away some of the excess heat. A lean mixture, on the other hand, burns more slowly, which tends to make the exhaust gas hotter. Side effects are less power, because the heat comes later in the power stroke, an oxidizing environment in the exhaust pipe, because not all the oxygen in the air is burned, and the ever-favorite blue chrome because of higher pipe temperatures.