4hp Briggs & Stratton Lawn Mower won't start?

I just bought a used Yard Machines by MTD w/a 4hp B & S engine. It ran Ok at first, then started to run rough and finally quite and won't restart. I put on a new air filter, spark plug and even tried a new carburetor. It still won't fire. The plug is dirty when I looked at it recently, i assume this is from it not running properly, but I am out of ideas on what to look for next. Please help with any ideas. Thanks!!!