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5200 is pretty low for that engine....

Portable fuel tank, line and bulb, connected directly to the pulse/diaphram fuel pump (and a fresh fuel filter) would be a good starting point.

This will allow you to say the problem is "inside"...

Problem is one of the following:

-Inadequate fuel supply in the VST
-Restricted INLET screen to fuel pump
-Weak Pump (or leaking pump outlet seal)
-Pressure Regulator Problem (least likely).
...

I think you're talking about checking for excessive VACUUM on the supply side of the system. That's really the only place you would "tee" in.

Fuel PRESSURE is tested at a schrader valve, located...

Better re-check those lines. Should be A1-15 from tank to bulb, and then 5/16" ID B1-15 EPA (Mercury Line) from a Mercury/Quicksilver bulb, to the engine.

Fresh fuel filter, and Pulse Fuel Pump...

You will need to check the fuel pressure on the water, under loaded conditions (at or above the RPM's where the problem is occurring).

Steps to follow will depend on the actual pressures.

Also,...