I've got a lawn mower with a B & S 4hp engine that my wife's grandmother gave us. It hadn't been run in 6 months or so and it wouldn't start. I drained all the gas from the tank and put new gas and carb cleaner in it. Checked the oil and it was empty. Put SAE 30 oil in. Started up fine except was running rough and spitting white smoke. I thought the smoke might be from tipping the mower during the repair process and the rough running from the carburator being dirty. I let it run for a few minutes and the smoke stopped and it started running smooth. I stopped it and went inside to get something. When I came back out, it starts up fine, but it's running really rough again - revving high then low then high then low in a pattern and shaking just a little. Also, a little bit of black smoke from the muffler - barely noticeable. I don't understand as it was working well. The only thing I did was add some more gas as I had initially just put a little in while I was working and didn't want to have to stop halfway through mowing to refill.