Have an 18 hp 42" cut craftsman riding mower (Briggs and Stratton engine) model 917.273762. The problem started three weeks ago. I was mowing and after an hour or so the engine started to bog down like it was getting too much gas. If I idled it lower it would continue to run. When I would ty to get it to run at normal speed to mow it would bog down again. Two weeks ago it started and ran fine for 45 minutes before it would once again bog down. (like the engine was flooded). It would continue to run at low idle. This week, the same problem after a half hour of mowing. It's not the gas because the borrowed mower is running fine with the same gas. I've changed the fuel, air, and oil filters earlier this spring. I changed the spark plug (all oem parts). It's like something is getting too hot and shutting it down. I don't know... help...