Pancake
Couple of thoughts on your fuel pressure observations.
1) 3/8" return line is just fine - I run 3/8" return line in my setups without any issues. TB fuel pressure regulator does not work well against return line back pressure.
2) Fuel pressure should be measured right next to TB inlet port. Take a look at my video:
3) Vacuum leaks do not affect fuel pressure. Fuel pressure gauge may have a air pocket that effects displayed values. -6AN = 3/8" should not affect fuel pressure.
4) Clogged Fuel filter or fuel sock - that may affect fuel pressure and volume.
Can post photos of your setup - how you routed fuel lines???
//RF
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"The Beast"
1975 Chevrolet C20 longbed
350/
700R4! with 3inch body lift
Dual Flowmasters Super
40's!
TBI retrofit completed (2007-07-29)
New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17)