Small Engines (Lawn Mowers, etc.): craftsman 19.5 briggs mower, wd40, airfilter

Question
About a 98 model 19.5.  Been a geat mower.  Here is the deal.  Will not stay running without the choke either all the way out or half way out.  Change the in line filter, cleaned the airfilter out.  sprayed wd40 around all the spring/levers/cables around the carb.  Still will not run without choke being out.
If you run it long, it will evently slow down and quit, then back fire rather violently.  At a loss and have no clue what do to do now.  Any suggestions?

Answer
Most likely the problem is a dirty carburetor.  This is the most common problem we see.

I would start by removing, dis-assembling and cleaning the carb with compressed air from an air compressor and using some carburetor cleaner.

You carb is probably not adjustable.  These newer carbs are very sensitive to any debris inside them.  A small dropplet of water can cause major fuel delivery problems.

Let me know if this helps.
Eric