There's about 1.5cm deadzone in my throttle.

I have taken it to 2 mechanics, they opened it up and lubricated stuff, but it didn't help.

The service manual expects a dual-cable setup, but the previous owner replaced it with this.

Do I tighten it at the throttle body? Do I buy a second throttle cable and run it? A mechanic recommended against that because it could catch on things.