Small Engines (Lawn Mowers, etc.): B&S #252707 engine problem, briggs stratton, supply pipe

Question
I have Briggs & Stratton Model: #252707 / Type: 0621 / Trim: 91031511 on my mower and today, after years of reliable service, it blew white smoke out the exhaust pipe and refused to start. After removing the air filter, it reluctantly fired up and ran rough at idle speed. It appeard to be wet and black inside the carb barrel though. It will not run with the (clean) air filter attached and squeezing the (new) fuel supply pipe seemed to help keep the engine running. Any ideas please?


Answer
Check the oil level.  Is it high, low or correct?  Does the oil smell like gasoline?

Remove the spark plug, crank the engine and see if anything comes out the spark plug hole.

If the oil is alright, replace the spark plug with a new one and see if the engine starts.

When was the last major tune-up?
Let me know what you find.
Eric