I just got a 2003 Yamaha 2-stroke 25 hp outboard. It starts fine and seems to run fine as well. The problem is that when the motor is shifted into forward and then the throttle opened up enough to get the boat on plane, there's a loud "banging" sound. This only happens when accelerating hard. At low speed, no problem. I noticed the tilt rod pin (used to set the tilt/trim of the motor) is missing. Any chance this could be the problem? It sounds like an "external" problem and not a problem coming from inside the engine.