I have an older (1980ish) 115hp V4 Johnson outboard. I start and stop my boat for short fishing drifts constantly with no issues. If I try running it up river for 15 minutes straight I lose all power and have to pump my fuel bulb repeatedly for the next 10 minutes before she will run smooth again. It doesn't stall out while I'm pumping the bulb. It continues to run but looses power constantly. Thoughts?