1998 Johnson Evinrude "EC" 90, 115 SPL Service Manual, P/N 520209, Page 207Get this manual
Trim Tab Adjustment
11 Improper trim tab adjustment can cause difficult steeringA propeller will generate steering torque when the propeller shaft is not running parallel to the water's surfaceThe trim tab is adjustable to compensate for this steering torque
Important single trim tab adjustment will relieve steering effort under only one set of speed, motor angle, and load conditionsNo single adjustment can relieve steering effort under all speed, motor angle, and load conditionsIf the boat pulls to the left or right when its load is evenly distributed, adjust the trim tab as follows:
With the motor shut OFF, loosen the trim tab screwIf the boat pulled to the right, move the rear of the trim tab slightly to the rightIfthe boat pulled to the left, move the rear of the trim tab slightly to the left