Briggs Carb/Diaphragm issue

[MountainFan]

I have a push mower with a B&S Quattro 4 engine (10A902 2139 E1). Trying to resurrect it after sitting for several years at my father's house. I have thoroughly cleaned the carburetor, installed a new diaphragm & gasket, and reassembled everything. Got it running and mowed the yard. A few days later, I tried to start it, but the primer isn't pulling gas. Took off the carburetor, repositioned the diaphragm and gasket, reassembled. Ran great and mowed a little bit. Next day, I tried again, but the primer isn't pulling gas again. Why is the diaphragm shifting each time the mower is used?

I've enjoyed learning small engine repair on the fly, but I'd like the mower to be reliable as well. I tightened all the carburetor screws pretty well, but was afraid to overtighten them. Any thoughts would be greatly appreciated!

 

[ILENGINE]

The plastic carb will warp under the round tube going into the intake, that can result in diaphragm shifting and even getting sucked out from under the gasket, causing running issues.