With two plugs firing the flame front for each travels less distance than for just one. A 250 might benefit but a 50cc cyl diameter is so small anyway. The faster the charge is burned the more pressure will be applied to the piston head early in the downstroke. At very high RPM this could make a difference and you would be assured when the exhaust port opens combustion would be complete w/o fire exiting the port. If 4 strokes are run too lean the flame front speed is slow enough at car RPMs that when the exhaust valve opens it gets burned by fire passing over it