My big block has been bored .060 and has a 4.25 stroke. It has 781 heads that have been heavily ported and flow tested. The compression is at 9.5:1. The cam is a Comp solid roller with 242 intake and 248 exhaust duration at .050. .650 lift at valve. The carb is an early 850 Mighty Demon on top of an Edelbrock RPM airgap. . It idles at 1000 rpm pulling 11 inches of vacuum. Timing is 12 advanced at idle – 39 full, no vaccum advance. I know it should have vacuum advance but the Mallory distributer I have doesn’t have it. The plugs are always sooty and dark brown. When I’m driving, if I open the throttle hard it sputters then clears out and moves. Then it runs good if I floor it again. I shift at 6500. The plugs have run dark since I installed this carb. Floats are set right and fuel pressure is right. I’ve heard the early Demon carbs were jetted rich. Would a vacuum advance clear this up or should I concentrate on the carb?