The pressure drop numbers you got look to be a bit low and I don't believe it should have kept dropping.
I'm a bit surprised that you are only running 100PSI, mine runs at 140PSI.
From information that I have;
The A2 VA 7116 model has a unique circuit that exhibits a dependable cut-off, a
high single pipe lift capacity and automatic two pipe purging.
The pump circuit contains a positive displacement involute gear set and two
controlling valves.
APPLICATIONS
- Fuel oil #2 and lighter, not compatible for use with blends higher than 5%
biodiesel.
- One pipe system (two-pipe system possible).
PUMP OPERATING PRINCIPLE
The fuel from the gear set flows across the cone valve causing a pressure
differential sufficient to overcome the spring force of the diaphragm valve. This
causes the diaphragm valve to close and the fuel is then routed to the piston
chamber. When the level of this flow causes the piston chamber pressure to rise
above that of the opposing spring force, the piston opens and flow passes through
the nozzle. The piston spring is adjusted such that a given nozzle pressure can
be maintained while any resulting excess fuel is metered through the seal chamber
and then back to the inlet on single pipe, and the tank on two pipe installation.
Cut-off speed is regulated by adjusting the cone valve to a level which gives the
necessary pressure drop to open the diaphragm valve. This in turn reduces the
piston chamber pressure and the piston shuts off the nozzle.
Improper nozzle cutoff;
Leak at nozzle adaptor ................... Change nozzle and adaptor.
Partially-clogged nozzle strainer ..... Clean strainer or change nozzle.
Tighten intake fittings. Tighten unused intake
port and return plug.
Air leak in intake line .......................
Run burner, stopping and starting unit, until
smoke and after-fire disappears.
Air pocket between cut-off valve
and nozzle ......................................
Strainer cover loose ........................ Tighten four screws on cover.
Filter leaks ...................................... Check face of cover and gasket for damage.
To determine the cause of improper cut-off, insert a pressure gage in the nozzle port of
the fuel unit. After a minute of operation, shut burner down. If the pressure drops from
normal operating pressure and stabilizes, the fuel unit is operating properly and air is
the cause of improper cut-off. If, however, the pressure drops to 0 psi, fuel unit should
be replaced.
NOTE: The A-70 pump circuitry is designed to give a high cut-off of superb quality.
Never use the amount of pressure drop as an indication of the quality or speed of
cut-off.
