Small Engines (Lawn Mowers, etc.): 5hp Briggs running problems, machine shop experience, briggs and stratton

Question
I have Yard Machine tiller by MTD with a 5hp Briggs and Stratton motor. I have a problem that really has me stumped. I have a background in engine repair as well as automotive machine shop experience and this litter bugger is about to drive me crazy. Well I pulled out the tiller the other day to get ready for spring and it would not start so I put a little gas in the carb and it ran for a few seconds. I though "hey its the diaphragm pump" so went to the local B&S shop and picked up the Dia. with a few new gaskets. Well, took off the carb, cleaned everything well making sure all the holes were open, filters clean, tank clean ect... and put it all back together. Well same problem, put a little gas into he carb and it ran for a few seconds. Nothing I have done to date has made a difference, including pulling the head, checking the camshaft, replacing all gaskets, making sure all bolts were tight. So I am stumped. Any advice would be welcomed.
Thanks,
Alan

Answer
Is the gas good?  If you remove the filter from the carb and place your hand on the carb throat to seal off the carb and pull the string will the engine start?  Sealing off the carb throat will force the engine to draw fuel from the carb.
Let me know.
Eric