1979 Bayliner Gears

My boyfriend has a 1979 17ft Bayliner. He has to replace the gears I believe. He bought a new handle and it came with 2 long cords that control the forward and reverse. He is having a hard time of adjusting them so both forward and reverse work. At one point only the forward worked then he adjusted it and now only the reverse works. Can you please help us? Thank you. My email is cheryls@wmccinc.com