Four Stroke Series - Portable Features & Benefits Our Three Year Limited Warranty protects every Four Stroke. Yamaha's Shallow Water Drive keeps the prop and lower unit out of harm's way in shallow water. The Microcomputer Ignition adjusts timing to maintain optimum engine performance, economy and durability under all conditions. The Fresh-water Flush Device enables you to flush the engine without starting the engine. Yamaha's Exclusive YDC-30 Alloy is the most corrosion-resistant base material in the industry. The Multi-Function Tiller Handle, optional on F25, gives you fingertip control of starting, shifting, steering and throttle. All Portable Yamaha four strokes exceed Federal EPA 2006 emission requirements and have earned a CARB 2-Star rating for very low emissions. Yamaha is Dedicated to Helping create a cleaner environment through advances in outboard technology.