Try this. Remove the regulator and disassemble it. Clean everything with glow fuel. Make sure to rub the diaphragm between you fingers while covered with fuel. Do the same with the plunger. inspect the area around the hole that the plunger fits into. Now assemble the plunger and pin through the hole in the regulator and make sure the pin is completely inserted into the plunger. Now slip the small end of the spring over the nipple on the plunger and screw in the adjustment screw. I try blowing into the small hole next to the diaphragm and see if the plunger is holding. One hole just goes into the diaphragm chamber but the other should hold pressure until you press on the plunger pin. carefully replace the diaphragm and tighten the regulator back onto the engine. I bet that fixes you problem.

Blessings, Terry