hi I have a 20hp single cyclinder kholer courage engine on my mower its about three years old and been took care of and allways run fine but latey after you mow about 21/2 or 3 hours its starts to run bad its starts to bogg and pop and run rough and I can choke it some its runs fine again as long as you keep choke on or I can just park it and cut it off and next day its runs fine till about same hours mowing and it does it again! I change plug, fuel filter, air filter and still does it, and ideas?