I have an 81 as well, so I may be able to shed some light.  This happened 
to me once when I replaced the gas tank without rinsing out the donor 
tank first.  Pull the fuel filter after clamping off the hose from the 
tank.  You should be looking at a big, square, white filter.  If not, 
just drain and pitch it, the PO put in the wrong one.  If you are, try 
blowing through it in the same direction the arrow's pointing.  If you 
wind up looking like Dizzy Gilespie then you've found the problem.  
Should be $5 - $10 at a FLAPS.

If the filter's ok, the easiest place to check gas flow is the gauge 
hookup at the left side of the engine.  There's a tee in the metal line 
to hook up a long piece of hose to.  Put other end of hose in a clean 
glass container and have friend (not girlfriend, she'll be no help 8) ) 
turn key.  If you get plenty of fuel, start looking elsewhere.

Write if that's not enough.

Jack