My hand throttle keeps slipping on my zetor 6211. It feels very tight and I have to hold it up with cloth stuffed underneath. It was recently fixed by a mechanic however is acting up again and I can’t get through to him to find out the issue. No new parts were required so I guess it was something basic. I tried adjusting the throttle clamp but when I start the tractor the throttle goes back to square one again. Any ideas?