If its rich at idle and you have correctly set the idle mixtures then double check the float levels. Incorrectly set float levels will effect the entire RPM range. Its also possible there could be a problem with the power valve circuit. I'm not sure how the Demon carbys control the power valve circuit. Its somewhat common for Holley's to have a split in the power valve diaphram or incorrect size valve and cause similar issues. Leaking emulsion block and base gaskets can also give the same symptoms.

It might be as easy as dropping down a couple of jet sizes in the secondary's to clear up the top end. I sometimes muck around with the vacuum secondary spring tension on Holleys to get them to cut in a bit later in the rev range to stop early rich under load situations. I would start with the float level checks first, then set the idle mixtures and timing and see if that clears up the rich mixture from idle to mid-range. If not then investigate the carby internals more closely, example power valve, gaskets and correct external plumbing connections . You can also check with the engine idling around 750 RPM and look down the carby venturis and see if you can see any fuel entering above the butterflies primary's and secondary's, if so you have a problem.

The above checklist is only a couple of common areas that can cause the problems your experiencing but you have to start somewhere.