I have a 93' Murray rider with a Briggs 12hp motor. Model 284707, Type 0135 01, Code 9505154E. It has a Walbro carb, 5-4993. When cold it always starts up and runs great. After 15-20 min of cutting it starts sputtering and dies, I can start it choked but it runs bad and dies again. If I wait 15 min it starts up and runs great until the same thing happens again. Multiple plugs have been tried (Champion and NGK), Got into the valves and ground off a few thousandths off the valve stems to make sure I had proper clearance between the lifters to valve stem (both valves) for when the motor reached operating temp. Carb looked very clean. Sanded down the carb flange on a tabletop with sandpaper. In case there was some warping of the carb flange where it bolts to the intake elbow. Nothing fixes it. Any ideas?