I have an 87 Wolfy with a rebuilt WBX (8000 miles).  It runs strong and
smooth and has no hiccups, cold/hot start problems, or oil/water
temperature/pressure problems.  It just gets bad mileage.  Before the
cylinder gaskets blew the OEM engine had 165k miles on it and got 19-20
miles per gallon.  Now I get 15.6 miles per gallon every time I fill up.
City/Highway makes no difference.  The engine was rebuilt properly; new
heads, cylinders, rings, crank, bearings, gaskets, etc.  In addition the
O2 sensor, coil, plugs, wires, cap, rotor, fuel pressure regulator, air
filter, were replaced.  What is weird is it does seem to run rich only
because I get a thick sooty deposit in the tailpipe, which I had never
seen before.  I have run the engine at all different RPM's and the O2
sensor (I have a test lead spliced in) shows a short swing dead center
on .5 volts.  You can even remove the fuel pressure regulator vacuum
line and watch the ECU immediately adjust based on the O2 sensor input.
Should I drive with the Multimeter attached and watch what happens to
the voltage under load?