1996 Mercury Force 25 HP Service Manual 90-830894 895, Page 15Get this manual
2Boat Bottom: For maximum speed, boat bottom should be nearly flat plane where it contacts the water and particularly straight and smooth in fore-and-aft directionaHook: Exists when bottom is concave in foreand-aft direction when viewed from the sideWhen boat is planing, "hook" causes more lift on bottom near transom and allows bow to drop, thus greatly increasing wetted surface and reducing boat speed"Hook" frequently is caused by supporting boat too far ahead of transom while hauling on trailer or during storagebRocker: The reverse of hook and much less common"Rocker" exists if bottom is convex in fore-and-aft direction when viewed from the side, and boat has strong tendency to porpoisec Detonation generally is thought of as spontaneous ignition, but it is best described as noisy explosion in an unburned portion of the fuelair charge after the spark plug has fired Detonation creates severe, untimely, shock waves in the engine, and these shock waves often find or create weakness: The dome of piston, piston rings or piston ring lands, piston pin and roller bearingsWhile there are many causes for detonation in 2-cycle engine, emphasiS is placed on those causes which are most common in marine 2-cycle applicationA few, which are not commonly understood, are: 1Over-advanced ignition timing 2Use of low octane gasoline3Propeller pitch too high (engine RPM below recommended maximum range) 4Lean fuel mixture at or near wide-openthrottle5Spark plugs (heat range too hotincorrect reachcross-firing)6Inadequate engine cooling system)cooling (deteriorated