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.
For what it's worth that is the dinkiest most Chineseium throttle assembly I have ever seen in my life. It looks even worse than the one that came on my Orion RXB.
Your deadzone is most likely caused by too much slack in the cable. If I were you I would deposit that throttle assembly directly in the trash and replace it with an OEM or appropriate OEM equivalent one. Kit-bashing a single cable throttle on a throttle body designed for a dual cable is not a good idea, primarily because the second cable is explicitly a safety feature. Its function is to allow you to positively force the throttle closed in the event of a return spring failure or any other stickage kind of situation. You can take my word for it on this one, you do not want to be in an unexpected throttle open failure situation on your 142 horsepower sportbike.
There should be no risk of the second cable "catching" on things. As configured from the factory it runs along the same path as the existing throttle cable, immediately next to it. If it's going to "catch" on anything, so is the throttle cable you already have. From stock they were probably even tied together at a couple of points with those weird OEM zip tie things. Part two of my advice therefore is to fire your mechanic.
Your opener cable is part number 54012-0601 and your closer cable is 54012-0602. (via) Your handlebar throttle assembly is a combination of 46019-0020 (the grip part) and 32099-0735 (the casing).