I replaced both head gaskets,adjusted the valves to .004, changed the oil, plugs, and air cleaner. It started right up and I mowed for about 20 min and it started to smoke and blow oil. I discovered the dip stick tube seal blew. I since replaced the seal and readjusted the valves, checked the plugs, and now it won't start it turns over and backfires through the carburetor. Help Please!

Hi,
Could not say why it happened, but the most common source of your description is a sheared flywheel key.
Peace,
Clarke