My OS40 stalls every time it is nose down and

Nose down and what? Adjust top end and check air bleed adjustment per manufacturer paperwork. Air bleed is opposite in adjustment. Out is leaner and in is richer. Good luck, I'm assuming it is a LA series motor

Sounds like a fuel pickup prob in the tank.

Check your internal tank fittings -- vent & pickup.

The vent tube should extend to the top of the tank & the fuel pickup obviously has the flexible hose & clunk weight.

Make sure that you aren't trying to feed from the vent tube. Also make sure that there are no pinholes or tears in the pickup hose & that the pickup line allows the clunk to swing freely without binding on the back end of the tank.