Re: 1978 115 mercury need help on disconect of cable
The throttle cable (Top) comes off first. Remove 7/16 nut that holds cable end to throttle arm. Open the barrel nut clamp, by gently lifting on on the tee handle and swinging it in, up and out. Now pull out the flat part of the clamp. The shift cable end must first be unfastened (7/16" nut) from the shift slide. Just loosen the nut and push the aluminum clamp toward the engine block. The cable end should just clear. Now you may remove it's barrel nut end.
Re: 1978 115 mercury need help on disconect of cable
You do not need or want to remove the black plastic end of the cable from the cable. If you loosen the set screw the cable end will pull out. This is not necessary to do.
The end of each cable has a hole where it attaches to the motor. I presume you removed the throttle cable? Both cables look the same. The shift cable has a "Peanut" shaped piece of aluminum covering the end of the cable. If you loosen the 7/16" nut in the scenter of the peanut, you can then slide the peanut off the end of the cable and it will be removed.