I picked up a 2005 Merc 15 a couple weeks ago. When the seller test ran the motor for me it started right up and ran good once it had a minute to warm up. I took the motor home and discovered the gas in the tank was bad. So I got fresh gas and again the motor ran like it should. The next day I took the boat out and right away I had trouble getting the motor started. Once started it won't idle and at full throttle the motor isn't running at full song. Sounds like its missing something, don't know too much about motors to really tell you whats happening. I ran the motor off and on for about 4hrs and it didn't get any better. I hope its not something serious. Any thoughts?