Sounds like Regina injection, from the idle variation. Most of them do that, and although it isn't "right", it doesn't necessarily mean that the 02 sensor is totally dead.

It could be a fuel pressure regulator issue, but usually (not EVERY time, but usually) the problem is as invar said. The pressure membrane gets a leak and dumps raw fuel into the intake manifold, which of course ruins fuel economy as the pump dumps 1/3 again as much fuel into the engine.

The puff of black smoke isn't unusual at higher RPM and wide open throttle, as the high velocity of the exhaust blows a lot of debris from the exhaust side, from the manifold to the exhaust pipe. Still, the FPR shouldn't have wetness inside the vacuum side. It WILL smell like fuel, however slightly.

Assuming that you have a new air filter, new spark plugs set to .028-.032, new cap and rotor, and good spark plug wires, you might very well be seeing a bad fuel pressure regulator. HOwever, if any of the above parts aren't right, then you might have an issue having to deal with ONE of the other components... or maybe more. Typical tune up stuff.
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1992 940 wagon, low miles
as well as others.