I work with a lawncare company and he has an exmark navigator with a 27hp kohler motor on it. The mower has 975 hours on it. The mowers starts in the morning fine and after about 5 minutes of mowing it starts to bog down and if you turn the chokle on it will keep on running. After a few more minutes it will barely run on the choke. I shut the mower off and wait 30 to 45 seconds and start it up and it will run fine on no choke for about 5 minutes again and then do the same thing. Any help would be grateful. It is very frustrating.