1985 OMC 65, 100 and 155 HP Models Commercial Service Manual, PN 507450-D, Page 323Get this manual
Trim Tab Adjustment155 The boat may be slightly easier to steer in one direction or the other due to steering forces or other reasonTo check this, run the boat in straight line with balanced load in an area where current and wind will not be factorTo obtain equal steering effort, adjust the trim tab as follows: 1Turn the steering wheel in both directions to determine the direction that requires the least amount of steering effort
2Loosen the trim tab bolt and if less steering effort is required to port, move the trim tab slightly to portIf less steering is required to starboard, move the trim tab bolt slightly to starboard
3Reference numbers, thru 6, are embossed on the trim tab