Typically the low pressure FP's, [4-7psi] don't require a pressure regulator. With pressure fluctuations at temperature, Holly recommends a by-pass regulator, [p/n: 12-803BP]. Is this the one you have? The regulator should be set to 7 psi, 4psi is too low. Also, were is the fuel filter mounted and what micron is it? To fine a filter on the input side of the pump may restrict flow. A 100 micron filter may be used on the input and then a 40 micron before the carb.
re: black/wet plugs; this may be caused by a leaky needle valve in the carb.
