Walk Behind Lawn Mowers

I have a HU775EX that I bought from Lowe's about 3 years ago. It worked great for the longest time. Around August 2013 the mower went into storage until March 2014. I drained the gas during this time so it would not go stale. The mower would not start at all since then until I put a new spark plug in it. I then put new gas back into it (with ethanol treatment as always) and the mower will run for a few minutes, then shut off. I cannot restart the mower for about 30 minutes after that. When it shuts off and I try to restart it, it will sound like it's trying to start, blow out a puff of white smoke, then die.

I took it to a repair shop yesterday and got it back today. The mechanic replaced the air filter and spark plug, but it did not help at all. The spark plug was completely covered in carbon, which is weird since I just replaced it about a month ago. Could old/bad oil be doing this? I've read that the automatic choke is one of the biggest problems with this mower, but the mechanic checked the choke and said it's working fine.