Yesterday we took the pontoon boat out for the third time this summer. Two previous days we had ethanol free gas. Yesterday, we had to use regular gas but put the marine gas additive in. After an hour of cruising around, the motor suddenly started to vibrate intensely while at full throttle (had been at full throttle for 10min or so and had been fine) and the boat slowed down. So I throttled down, and it was fine... for a few minutes then started vibrating/shaking the boat again. The whole time RPMs were steady. Prop looks fine as far as I can tell and is not loose. Ideas of what this could be? Prop imbalance, gas ?? Any assistance would be most appreciated!
