The main problem symptom on my Yamaha 2003 Z250TXRB is that I cannot get acceptable RPM at full throttle; the most I can get is 4100 to 4200 RPM. I also experience rough idle and stalling at low forward speed. I replaced the spark plugs and there was no change in the symptom. I took it to a shop and they said that the compression tests were normal and that diagnostic tests were all normal on tests at the dock. But I showed them there is one cylinder that was not working (this was pointed out earlier to the facility). I know this because the porcelain on the plug is new white. They connected the diagnostic equipment and ran that boat at high RPM and said that the ignition coil was breaking down. The coil was replaced. I took delivery of the boat and on my first trip, I noted the same problems. I examined the spark plug the porcelain on the plug is new white. The plug is the lowest plug on the port side. Close examination of the spark plug in question indicates that the electrode is darkened indicting that sparks may have existed. Does anyone have any idea what could cause one cylinder not to work?
