I recently purchased a 67 fastback with a 93 roller 302 with GT40 heads and a 650 Demon carb. Anyhow, it has been working fine up until today. I started the car up and it seemed to be running fine for a about 20 seconds. I reved it a couple of times and then the car idled low before dying. I restarted the car afterwards and had to give it a bunch of gas before it started idling really rough and rich. Lots of smoke and had to have the pedal depressed pretty far without it stalling. Over the next couple starts it only seemed to get worse before I gave up. I gave up and didnt want to make it any worse, so I pushed it back into the garage.

Any ideas?

 

Sounds like you might have a stuck float. Demons have clear site plugs on the bowls. Can you see fuel through the plug? Remove the plug from each bowl (without the engine running) and see if fuel pours out indicating the float is stuck.