Small Engines (Lawn Mowers, etc.): B&S 4hp lawn mower engine wont start, dipstick tube, crankcase breather

Question
I just bought a used Yard Machines by MTD w/a 4hp B & S engine. It ran Ok at first, then started to run rough and finally quite and won't restart. I put on a new air filter, spark plug and even tried a new carburetor. It still won't fire. The plug is dirty when I looked at it recently, i assume this is from it not running properly, but I am out of ideas on what to look for next. Please help with any ideas. Thanks!!!

Answer
Did you check the oil?  If the more has been Ron low on oil in the past.  You could have internal engine damage.  When you replaced the carb, did you clean the entire fuel system?.  That includes fuel line tank screen inside the tank and the fuel can't get if there is a restriction in any one of these this could happen.

This also could be caused by a air leak in the crankcase, an air leak will develop at the crankcase parting halves, at the dipstick O-ring and the dipstick tube O-ring were the dipstick tube goes into the crank case.  Also you could develop an air leak at the upper oil seal under the flywheel or the lower oil seal, under the engine.  If it's on the bottom you would see whale on the top of the blade.  

When you say the engine ran rough and died did it smoke when it was running rough.  If it did it could be a crankcase  breather problem.  The crankcase breather is a small rectangle or box fitted into the top of the crankcase.  There is a small rubber tube that comes out of it and vents to atmospheric pressure.  Check to see if that is filled with oil.  If it is take out clean it so it's clear and put it back.  Hope these suggestions help please let in