If the cable moves relatively freely and not any major binding I'd get a cable lube tool and can of lube. And start there. If it feels like it's binding up replace it. Last thing you want is for it to stick wide open. At any time. WSM performance and SBT sell cables.
When set up correctly. The throttle trigger should be against the hand grip as the carburator butterfly is wide open. You don't want the cable being pulled thight before trigger is hitting handle bar. This can damage carb and cable both. Then when the throttle is released there should be about a 1/4 to 3/8 of an inch free play.
Pic is of cable tools.
Thank you. On the 500 it's got a catch in it. The 650 seems pretty smooth. I have new cables coming for both. I'll only change them if needed. I'm just confused as to why I can't get them loose enough for the throttle to close