What causes oil in carberator, briggs and stratton

Oil in carberator briggs and stratton

I have a mtd tiller with B&S engine, when you start it, it will run a few seconds, then blows oil in carb. The oil is coming from tube from valve cover to carberator. I replaced the head gasket but it is doing the same thing. Any suggestions?

First check the oil level.
Then check air filter make sure its good,
Then check any oil leaks,
Luck