Marie,
 
First order of business: "he who has not sinned, cast the first stone" .... or something along those lines. ;>) not that not being active on THIS list is any kind of sin ..... likely more like an admirable virtue! ;>)
 
I've had an I4 (TIICO) in my van for a little over 5 years and have gotten up to just under 24 mpg, to as poor as 17. The worst mileage comes from a lot of mountain driving (altitude + steep climbs in 2nd/3rd gear) and headwinds on highways with full camping load. 
 
Does your engine set up have an oxygen sensor on the cat/exhaust system? I'm thinking that if you do and it's not working, that your van is running on "limp-home-mode", which will allow the van to run but won't make any fuel delivery adjustments that it normally would. The result is usually exactly what you report, rich running, black plugs and poor mileage.
 
Bob
'87 Syncro Westy> > I suspect that my van is running a little rich....my sparks plug tips are black, dark black....and I am getting around 17,6 mpg.> > Does anybody want to share their average mpg?> > Thanks a million!> > Marie
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