I have a 6.5 hp Sears mower (large rear, small front tires-don't know model number, but trust not essential?), about 3 years old, which has given me very good service, including starting (most of the time) on the first crank. Air filter is cleaned, spark plug and oil changed regularly, etc. About a month ago, I wanted to mow, but the mower uncharacteristically just did not want to start. Finally, I primed it several times and it burst into life. I did my mowing, but when I released the dead-man bar, it died as it is supposed to, but unusually with a considerable burst of smoke (dark colour). I tried to restart it without any success until I again primed it several times (maybe a dozen) and it started with a cloud of smoke but immediately died. Could not repeat the start. Changed the old plug for a new one, removed the carb and checked/cleaned the float chamber. re-assembled with high hopes, but no cigar. Finally changed the air filter and the gasoline and it started again. Dirty gasoline maybe? Have used the mower once since getting it running again (it ran normally), and today is second. It started and ran normally, but when I shut it off to move to a different place, the dark cloud came from the exhaust again and now it simply will not start. I have to believe that the dark cloud is the key to the problem, but have no idea what might be causing it (oil level in within markings and I was mowing level ground.) Since I got all new gasoline a month ago, I do not expect that it is now a dirty fuel issue although anything is possible. However I am adding Stabil to the fuel at the recommended amount and wonder if this could have anything to do with this problem? If not, what should I be looking for? Thanks. P.S. If I remove the fuel from the mower tank, what could I use to filter the fuel to see if there is any water in it (would like to rule this out)?