OMC Zephyr sail drive loss of power
Aside from the obvious bad fuel or bad mixture you might also want to check the water intake to make sure it is not obstructed. The water intake is located forward of the propeller on the bottom of the drive and the and the water supply valve is located just above the drive unit mounting plate below the air silencer. Good luck let me know if this helps. Often times bad fuel or mixture will result in carbonizing even new plugs to the point that new fuel doesn't seem to correct the problem.