I have a 2007 Yamaha VMAX 150 hp. I have never had any issues with the engine. Yesterday as I was cruising along at about 27 mph or 3300 rpm the motor noise went from normal to a somewhat throaty sound. I backed off plain for a bit and checked for water in the fuel and made sure the spark plug wires were all on properly. Those items checked were all good. I restarted the engine and gave it throttle. It got on plain, but slowly and weak. The engine will not make it to full rpms or speed. When I got on land I removed the spark plugs. The top two left side spark plugs had quite a bit of unburned oil on them and the other 4 looked normal. The plug looked as if they were not firing. What could be the cause for this issue?