From your description I would still suspect the return line. Is there a
1-way valve in there?
A way to test it would be to connect the forward line (from the pumps)
directly to the return line using a T-piece with the differential
pressure measurement moved to between the 3rd leg of the T and the
airbox. Run a pump, then both pumps, and see the lowest differential
pressure the regulator can be expected to provide. If it is over 5 psi
it is out of Rotax spec and the return line needs repair.
It could be a very improvised connection - it should only have to deal
with a gauge pressure of 5 psi.
On the other hand, with a completely closed return line a pump delivers
1.8 bar (26 psi). And both in series 3.6 bar (53 psi).
In that case you can make it a very short test.
At zero pressure the pumps are rated to supply 120 l/h. I have heard
that the pumps tend to get better with use. Maybe the improvement to
your current 160 l/h is making it apparent that the return line is too
restrictive.
Regards,
Jan de Jong
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