Hoping you all can help.
We are a small nonprofit rehabbing a lighthouse. We recently had a boat donated. The engine on the boat is a 2007 Mercury OptiMax 225XL. the only back story we have on the engine is that it may have sat in the yard for up to five years. We did the basics: new oil, new gas, new plugs, new tracker valve, compressions, air pressure. When all stable, the engine started and ran, on the muffs. When we went to dunk the boat it wouldn't make enough power to get up on plane. Got to 3300 RPMs on a 15 P prop and would only barely push about wave out of the way. It seemed to idle evenly for as long as we wanted it to. we been noodling around among our own quasi-experts and getting way too many suggestions. Have started to read many of the things here and I'm very impressed with what I've come across.
Based on some of the suggestions, we ran the engine and pulled the injector plugs and spark plug covers one by one. for all three cylinders on the port side, when the injector plug is pulled, nothing happens. Engine continues to run, at idle, exactly the same. The same thing happens when we pull the spark plug covers. As we pull the spark plugs out, we could see sparks jumping out of the cover seeking ground (Not sure if that's supposed to happen either). In the first 10 minutes of running, any injector plug or spark plug cover pulled, on the starboard side, would stall the engine. After it warmed up, pulling either, on one cylinder, would simply cause the engine to run rough. Seems to us that all three cylinders on the starboard side are not working.
What's the checklist to determine why? Where we begin? Remember that this engine may have been sitting for up to five years.
Any help would be greatly appreciated.