To start with you should have returned it and got them to repair or replace it ,I know hind site is great... other than that it sounds like a burnt valve , also have you tired replaceing the spark plug ?? I'm sure you will get some good advice here these guys are great...

I am not going to recommend this due to potential liability purposes, but I know one person who has strapped down the "dead-man" switch, so he can just idle down the engine while he empties the bag.

However, even he stops the engine and lets the mower cool down and rests before he refills an empty gas tank.