Why Does Engine Only Run Well With Pressure Tester Connected??.

[DienstXIV]

Hey everyone, I have a 1975 911S and I have been having trouble figuring out why my engine runs poorly. I checked for vacuum leaks half a dozen times, replaced all vacuum hoses, all fuel injectors and seals, all intake runner boots and seals, I also replaced the fuel pump, cold start valve and had the WUR rebuilt.

My engine doesn't like to have a consistant idle. When warm the RPMs go up and down between 1200rpms and 2000RPMs, sometimes I can tune the idle screw and fuel mixture and get the RPMs to settle at 900RPMs but then I take the car for a spin and it totally changes. Sometimes it will decide to have a steady idle at 1200rpm or 1300 or it just will stall out or go back to going up and down but again. It's very irradic.

I decided to check the fuel pressure to see what that would tell me and see if the fuel pressure might be fluctuating as well. I hooked up the fuel pressure meter inbetween the WUR and Fuel Distributor and with the valve open the engine runs great! The idle problem is totally fixed but when I close the valve everything goes back to the same issues. Anyone know why this would be?

Btw control pressure is a hair over 2.7bar and system pressure is a hair over 4.5bar.

Cheers!
Alex