Check the IAB's to see if they are open. Demon's have a reputation for crud in the passages. If one or more of the IAB's had something in them, this would richen the idle circuit.
Which Demon? (Speed or Mighty)

Note: I had a 650 DP Speed Demon on my mild 340 and the stock jetting of 70/78's gave very good plug color. Best idle was with all four corners out 3/4 turn each.