[Baklava]

I have a cub 1862 with a kohler m18. Engine model# is m18s 24646 and the mower model # is 141681100 serial 814619. After ten minutes of running the rpms will skyrocket unbelievably high. And then you have to shut it down asap....Then once cool you can run it again until about 10 minutes pass and the rpms skyrocket again....I ruled out governor in my mind because its mechanical and it would not wait until warmed up to act up, at least from my knowledge and experience...so I thought maybe the carb needed a good cleaning...did that and still same problem....Could it be that when it warms up it is sucking in air somewhere into the intake? Or what are your thoughts my kind helpful people?
Thanks in advance......