Perhaps the black tailpipe results from the enrichment mode of the
engine.  Most, if not all, gasoline engines run rich at high loads.  The
engines are enriched as a function of throttle position or some
convolution of throttle position and air flow or straight out manifold
air pressure (not the VW Vanagon, it doesn't use Manifold Air Pressure:
MAP). Try putting in a good O2 sensor and monitoring operation, it
should read about ~0.4 volts at stoichiometric and ~0.8 volts at rich
operation ( ~0.0 volts at lean operation)  a new sensor should only cost
about $15. Try to get hold of an analog volt meter to do the measuring.

Remco

87 Toy MR2
85 Vanagon RIP