I have a murray lawn tractor with a briggs and straton 14.5 ohc ic engine. The motor starts right up but blowas a little smoke into the shed, it will run fine for about half an hour and then will start to stall and blow a huge amount of smoke out of the exhaust, I saw a small drop of oil on top of the exhaust this last time. If you try to start it up again right away just more smoke and rough idle. If it cools for 30 minutes or so you can fire it up and run again for 30 minutes or so depending on the heigth of the grass and the load the motor is under. Can anyone help with this or maybe tell me where I can buy a new or refurbished briggs motor model number 287707-0255-01. Thanks

Check your oil and see if its over full .
If so smell it see if it has gas in the oil.
If so change oil and replace needle in carb.
Luck