Man, all the talk about rich and timing etc for an off idle stumble is mostly silly. Does it have to have the correct (enough) advance yes, as with any carb. But Silver is most likely correct about the off idle stumble being a momentary lean condition. For the OP you must remember that the carb is a vacuum referenced metering device and each circuit in it has a job in a specific load range and RPM. This is why advance can be so touchy, up to a point increases in advance also increase vacuum. In EFI fuel is constantly controlled via pulse width from the CPU so any momentary lean or rich condition can be compensated for immediately. With a carb when you tip into the throttle the main fuel circuit cannot compensate for the increase in available air until the vacuum signal starts to pull extra fuel from the main/power circuits though the booster venturies. This takes a second or so. To make up for this momentary lean condition the carb is built with an accelerator pump circuit which operates solely off of mechanical movement of the throttle arm giving a squirt of fuel just as the carb is about to go lean. If the lever/spring adjustment is too loose or there is a blockage or other problem with the acc. pump system then the carb will go lean momentarily and if severe enough the engine can die all together. My advice is to go to the Demon web site; look at carb identification, then download the correct manual and read it cover to cover so that you understand your excellent carb's operation. Specifically, set-up, adjustments (I say Acc Pump) and trouble shooting.
Start on the right at chapter one - ID your carb. Good luck man and feel free to ask other questions as well.
Demon Carburetion - State of the art carburetors for the race and hot rod industry.