Running rich, low compression, misfire?

[Orin Eman]

A local 4ktq, MC conversion (no not mine) is running badly.

It seems to be misfiring on cyl 5.  Now cyl 5's compression
is admittedly low - around 95 psi, up to 140 if we dump
some oil in the cyl.

Anyone have experience with how low compression an MC will
run with or is there something else?

We've swapped cap and wires, the rotor is new.  Pulling
the wire from #5 plug makes no difference to RPM whereas pulling
any other has the expected effect of making it run rougher.
The injector has been replaced with a used injector that has a
decent spray pattern.

Mixture is such that the O2 sensor reads about 0.5 V.  Attempting
to lean it results in the engine stumbling with the sensor
still reading 0.3V.  Sure seems to indicate #5 isn't firing.

Any other ideas other than #5 is DEAD?

Orin.