How to replace starter rope on troybilt weed eater model no.tb90bc

Hi,
Because of the nature of the starting system on that unit, it is an assembly. Part #753-04459 It is a "Spring-Assist" starting system. Never seen one fail that was not either carb/fuel problems or operator abuse.
If the carb is working correctly, it takes almost no effort to crank; does not require a "sharp" pull as a regular two stroke and should never be pulled until the rope extends to full length.
I was amazed many years ago when explaining to a customer that it did not take pulling the rope out until it stopped to crank his machine. The customer denied doing it! At that point I showed him the stater and the remnant of rope with scarcely anything but a knot left, and asked "Just how do you think you can break it here otherwise?"
Peace,
Clarke

Weed Eater Instruction manuals are available for download from:

InstructionSheets.com - Weed Eater Instruction Sheets

Good luck