Unless the diaphragm is torn or if the rubber has gotten hard or stretched, I would not suspect the diaphragm. Otherwise they are easy to replace and at least you can eliminate it when troubleshooting. Tillotson DG-1HD is the part #.
Bob Japundza CFI A&PIA
N55BC RV-6 borrowed, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.