When it comes to two-stroke outboards, spray-type, chemical-based engine treatments are an important part of any maintenance regimen. Since piston rings do not rotate on a two-stroke piston, carbon deposits can form between the piston and piston ring. Once a ring sticks, the piston's sealing ability is diminished and performance suffers. If allowed to continue, it may score the cylinder wall or worse. Either way, you'll have to rebuild the powerhead. Chemical treatments should be used on a regular basis; check your owner's manual for the manufacturer's recommendations.