Lawn tractor stalls after a few minutes
If the smoke is coloured blueish white you are burning bit of oil. it could be many things. overfilling the crankcase with oil, Malfunctioning crankcase breather or air leak in the crankcase are the usual things to check. if the head gasket is sound and you are not operating the machine over 15 degrees tilt all the time that leaves only few other possibilities. simple ones like wrong oil type or water in the gas. You mentioned you check the carb and imo water in the gas is very high unlikely too. Worse case scenario is worn cylinder or rings if ignored can cause big problems. You probably done this already but if it was mine i would drop the oil fully and fill it with exact amount it says on the manual and give her few rounds around the yard with full power without the blades and than engage the blades and start cutting