This tractor starts and runs great but I seem to have an issue with the engine stop knob. There seems to be two idle settings, a normal slow and one with a very slight elevation (both with the same throttle position). If the tractor is in the slow speed the engine will stop with the cable. If it is in the elevated stage it will continue to run with the knob pulled out. If the tractor is run more than 5 minutes it won't idle at the slower seed unless you briefly put a load on it. It will remain at this lower speed until the throttle lever is increased. What could be the cause of this issue? Is there an adjustment for this? Thanks for your help and input.