Just an educated guess because Im not there in person, but it sounds like an actual fuel problem. I would check or just replace the gas with a New supply. Try a small mixed amount at 1st - assuming this is a 2 stroke engine. White smoke indicates that coolant is being burned, but if its an air cooled engine then there is none obviously. However, the fuel in the tank may be contaminated or just old as gas looses its chemical stability after awile. You can drain whats in the carb right now via the carb bowl drain screw, ( bottom side & might have to push on it as you turn if the screw is somewhat stuck in its port ) & put into a clean container to see it. The 1 in my avatar would not start 1 time. Power washed it & some water got into the tank through the tank cap vent. Examined fuel out of bowl, changed it & it worked great after. I dont think there would be a prob from the carb, but get the right gasoline : oil ratio if it is a 2 stroke engine.