Run problem 14.5hp B&S riding mower?

Cannot get it to stay running when starting. I've had small engine class and do most of my own auto repairs.
I've replaced the spark plug. Swapped out the gas solenoid on the carb bottom with a plain bowl, cleaned the carb. Found no trash in carb.
I recently replaced the flywheel ring and starter gear. I'm considering the flywheel pin has sheared but there has been no real strain to cause such and have not pulled the top shroud cover and flywheel nut to see.
I'm considering that a stuck valve has bent a push rod or damaged a rocker.
The engine has a good strong hit when the gas/air mix thins out after choking, so I don't think I have a bent valve.
At this point I'm willing to consider what others may suggest before I go any farther.

Rob Joyner
November 2008