No power in reverse 1988 Yammaha 70 hp

I replaced the handle gear shaft assembly on my 1988 yamaha 70 hp .No problems going forward.When I put it into reverse the engine goes into reverse, the boat will move slowly backwards but I can not increase the speed going backwards.

[boats.net]

There is an adjustment to allow more throttle in reverse in control box but you would have to disassemble controls to make adjustment.