I put a fuel pressure regulator on my 383 this week. Mounted it at an angle. It's almost horizontal, with the front facing downward.
My initial pressure was correct, then it went down to 3 or 3.5 . I'm trying to set it at 6.5 psi as my carb manufacturer requires, and it was there for a time. After it ran a while it started to show lower pressure. I'm wondering if it requires mounting upright.
I have an Airtex E84389 pump that's supposed to handle the required fuel. It's made for 454 carb engines. BTW there is one 90 degree elbow in my line,right out of the pump, like Mercruiser built the 350. Maybe I should turn the pump so there's no elbow, would that help?
Are Mercruiser fuel water separators a restriction?
Starting to have my doubts. maybe I'll bite the bullet and get a high end pump.

No there are millions running with no problem. I hope you did not put the separator after the pump..

Never ran a regulator different from how they are normally mounted. Doubt it would cause issues but , again, I have NO experience doing it this way.

Hi, turns out it had to run a little more, pressure did come up after some more use.

After setting it I always tap on mine a couple times to make sure it keeps the same pressure. It will do the same thing if you use it a little also. Guess the vibration helps them seat right or something.

Are you using a liquid filled pressure gauge? Those are notorious for reading off when they get warm.