Personally I would remove the carb, get it up on the bench and take a reading on what's inside. Look for junk in the bowls, make note of the jetting, and replace the power valve (a very common and inexpensive culprit in carb problems.) Make sure your float levels are good, replace gaskets and then take a go at tuning using Demon's literature. Also, some Demons have the idle-eze, see here:
Demon Carburetion - State of the art carburetors for the race and hot rod industry.
As Zoom said, tuning with an O2 sensor or sniffer helps cut to the chase.
Don Gould at 4secondsflat.com is a Demon expert. For $10 he'll send you his tuning guide, a must-have for getting the most out of Demon carbs. He also has a Demon baselining video on YouTube.