How do you change a throttle cable on a 1999 1100 rt motorcycle?

Hello Sherry i need you to go to the closest autoparts and have them connect the OBD2 computer scan you me and let me know what odes you have i can give you a better idea of where the problem is and what can be causing it

set the valve clearances and then balance the throttle bodies

There are two cables leading to the throttle. Both have two pieces.
Cable 1 goes from the throttle about 12" long and connects to an adjustment piece joining it to cable2 which goes to the throttle body.
Cable 3 also goes from the throttle and is about 12" long and connects to and adjustment piece joining it to cable 4 which goes to the spring on the throttle body.
To change any of them you must loosen the adjustment piece until the ends can be removed from it.
Then to change cables 1 and 3 you have to remove the throttle cover on the grip. you will be able to see how they are removed. Then reverse these steps to install the new cable.
To change cables 2 and 4 in no more difficult but more time consuming as you will have to route these longer cables from the adjuster to where they connect to the throttle body.
When cables have been install and all parts reassembled you can carefully adjust the tension on the cables until the throttle responds the way you like it.
Hope this helps

throttle cable is either routed wrong or just hanging up or pinched somewhere when installed.

Go to IBMWRA tech page , both these problems are addressed there. The gear switch in the tranny is bad and the throttle problem can be several things, TPS out of adj or TB's need syncing usually the case. Change the plugs to the Autolite numbers shown on the tech page.

I'm guessing the TPS is bad, located on the Throttle body. If that doesn't do it check fuel pressure, have had fuel pumps cause that problem esp if the bike has been in accident, cracks the fuel pump inlet tube causing it to lose pressure.