12Hp B&S engine won't run?

I bought a Tractor Supply HUskee riding mower used. It has a 12HP B&S engine (model 280707). It had been stored for a while so I replaced the plug/fuel filter, installed a mannual fuel cut-off & change the oil. It still wouldn't start, so I had to rebuild the carb (soaked in it dip 2 nights, replaced most parts. Some I couldn't get out even after the dip).

It now starts, but with a thunking noise (just one before it starts), puffs white ordorless smoke, and then cuts off after several seconds no matter how I adjust the throttle (Leave it in start & it quits, drop it down & it quits).

Any suggestions? Any questions before You could help?