318 power?

Swinger 340 said
I'm no expert, but tight or misplaced bearings could be the issue, if it is a new rebuild, if not, look into what you changed in the motor.

Yes, I've been thinking about that, but should the pressure really be so high at the filter then?
My friend who built the engine sadly passed away in November so I can't ask him. That's one of the reasons the project was put on hold.