Put a timing light on #1 spark plug lead. Warm the engine and have it at idle speed (650-700rpm). Point the timing light at the timing marks on the timing cover and front crank pulley. If it's not at 8BTDC, loosen the distributor clamp screw and rotate the distributor until it is. Tighten clamp screw. Shutdown engine and remove timing light. TB-IV doesn't have a 'base mode'. It's point and shoot.