As Neil said, check for sediment and water; I'd say drain enough to fill a mayonnaise jar, give it time to settle out, then see if it separates into gas and water layers.
  I'm beyting you've got a lot of water, and you'll need to drain a bunch out of the bottom of the tank, flush the lines and drain the carb bowl(s) completely.
  Even old gas will burn, especially when mixed 50/50 with new gas.  You've got to be sure it's mixed well.  Rock the car a bit to make sure, after you've drained the crap off the bottom.

Mike B.
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