Need help with Mercury OB throttle cable adjustment?

Here is a partial answer: The gear box has to lock the engine down when ever it goes from forward to neutral. This is done with the cross (trim) pin at the under side of the motor where the clamps finish up. Block the motor up a little and see if the two hooks that catch the cross pin are loose and moving when you move the throttle thru from forward to neutral .These hooks really suffer if the motor drops off a trailering bar when the shifter is in neutral or reverse.. Once this all works maybe your cable adjustment will take hold. And yes the br barrels is the only adjustment for the control cable. There are other adjustments on the engine which should be left to the mercury mechanic with a lot of referance books.