New sparkplug, fresh gas

Hi,
Had to assume a number of things to answer this. First I put a 536 vendor number in front of your model which brought up some things at Sears, but not the engine. I assumed a Tecumseh engine, so I went to a large engine distributor to get you a link to the manual. It is at:
Tecumseh Small Engine Specifications
And you should pick L Head Service Information to get the manual. It is commonly a dirty carb, but could be a crushed or punctured float.
Peace,
Clarke