Check retaining pressure. If drop in pressure detected, test pressure regulator by blocking off runback. Negative pressure regulator not connected. Runback pipe defective. Air-fuel control at limit / clogged (error Mechanical running noise. Possible origins of cause Pump mechanism worn.the apd diaphragm bypass regulators are compact lightweight units designed to regulate fuel pressure when using an apd mechanical fuel pump #6100 or other aftermarket belt drive fuel pump. the apd regulators feature a black anodized finish, with 3/8 npt inlets, a #8 an o-ring outlet, and a 1/8” npt fuel pressure gauge port.
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