Small Engines (Lawn Mowers, etc.): Engine problem or something else?, briggs stratton, hp engine

Question
First of all, I'd like to thank you upfront for kindly answering my question...
I have a lawn mower (5 years old) powered by  Briggs & Stratton 4.5 hp engine...
As of lately the mower running really rough, misfiring and
stops every 30 or so seconds emitting black smoke in process...
I changed oil and spark plug, clean the air filter and carburator to no avail.
The funny thing is when the mower is about to run out of gas (like last minute) it gradually pick up the pace and
start working normaly...

Answer
Hi Mitch
It sounds like you have a flooding problem. Black smoke usually indicates too much fuel or too rich of a mixture. Not knowing the model of your engine, I'll have to be general. It sounds like you have a sticking float or you have some clogged jets in your carb.
Hope this helps.