I recentley bought a pontoon with 75 hp Mercury 2 stroke. During test drive with previous owner, the motor started and ran like a brand new one. The previous owner had filled it with gas. I refilled it this weekend, engine started no problems. After running for about 30 minutes i turned the boat off. After several minutes i tried to start and it wouldnt start. several minutes later it started up and ran fine. I had no problems again and until I refuled again. This time It was hard to start, would not idle. While operating RPM where running higher then normal. Im mixing 50 to 1. and using premium gas. Any help will be apprciated.