4kQ, running rich

[Orin Eman <orin@wolfenet.com>]

Pete's 'old' 4kQ is running really rich.  Any ideas?  We have checked
the following:

Plugs are uniformly fouled - black.  New plugs didn't help.
Compression 140 to 170.
O2 sensor heater resistance OK.
Differential pressure regulator OK.
Temperature sensor OK (swapping for known good and vice-versa).
O2 sensor reporting rich.
ECU is pulling differential pressure regulator current
down to 0mA.
After a full turn of the mixture screw it is still rich
and running poorly, though the O2 voltage came down slightly.
(This is way too much to turn that screw in normal circumstances.)
System fuel pressure is 5.8 bar, differential pressure 5.4 bar
which is close enough given the JCW guage.
Cold start valve is neither squirting nor leaking.
The fuel pressure remains over 40 psi after shutdown.
The air flow plate is binding very slightly (hard to feel,
but there neverthelesss).

This was a sudden failure, not gradual.

It seems to me that the ECU and electronics are working
and that the problem must be mechanical - either the fuel
distributor or differential pressure regulator.

Any other ideas?

Please reply to 'all' to include Pete as well as myself/the list.

Orin.