B&S 3.5HP I/C rotavator engine?

This is a YBSXS.1481HH engine, about 15 years old but with little use. I can start the engine and it runs a while but as soon as I put load on it stops. It will only run with full choke but I can't see an air leak. I've tried altering the fuel screw with no result. The carb diaphragm is concave where the spring fits into it - could that be the problem? iI can't see a hole or crack in it. Plug is new and correct type