I have to say I'm intrigued by this post...a carbed Sportster should get about 42-45 mpg highway city maybe 36-38 mpg...at least my 2004 does with somewhat rich jetting, 2 washers under the stock needle, a #46 jet, (I think #42 is stock), 1 3/4 turns on the mixture screw, after I cleaned it throughly in 2008 when I got this visiting the HDT forum back then.
Just changed the old HD plugs with stock new ones, and put over 1200 miles a couple weekends ago and still averaged just over 40 mpg combined. My pipes have a little black soot and my plugs were coffee brown with 15,000 miles on them (I cleaned and gapped them once).
That fuel is going SOMEWHERE...make sure the enricheners needles are fully retracted when all the way in, check that charcoal cannister and plumbing, it may be full of fuel. Check the lower parts of the engine and shop floor for evidence of leakage, as well as the aircleaner (if it is gas/oil soaked, that may be suspect too). The intake will take that extra dollop of fuel and not affect running too much, especially if the evap system is also helping to void out the extra fuel as well even though mixture is way too rich.
