I went to cut my grass a couple weeks ago, and it would run and stall, with white smoke billowing out. I found it was trash in the carb...I just took apart the carb yesterday, and cleaned it thoroughly, and put new oil in the mower. Is there a chance there could still be a trace of gas in the oil? Is changing it once enough, or should I run it a few minutes, and change it again, to make sure it's all oil now?