Thank you for your answers. The vacuum line is good. I think I'll pop the stock regulator back on and see what it does.
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1990 Mustang GT, Holley Systemax, GT40X, B-cam, 24lbs injectors, Pro-M 80mm, 70mm PP TB, BBK LT w/X-pipe, Magnaflow catbacks, Baer, Eibach, Tokico, Walbro 255lph, UPR subs/uppers/lowers, 31 spline Mosers