I haven't played around much with Demon carbys, jetted plenty of Holley's & a few Rochester's, the principals are all the same for tuning. Is the Street Demon a vacuum secondary and have a square bore or spread bore, or is it a double pumper carby. ? If its a vacuum secondary carby you should be able to make it work or at least reasonably drivable.

Tuning or a good tune depends on many factors, compression ratio, cylinder balance, timing curve, manifold vacuum (particularly at lower rpm), cam type and overlap, exhaust system and engine condition. I was running a Rochester on my van with 253 manual mild cam and 9.8-1 compression. The spread bore Rodchesters are actually a 770cfm carby from memory.

However hear are a few things to try first.

If it's blowing lots of black smoke under acceleration at low to medium RPM then I would start by reducing the size of the power valve circuit, it will have a valve or jets. If its smoking under quick acceleration then clears up slightly then look at reducing the accelerator pump circuit. This might be as easy as adjusting the linkages to reduce stroke travel on the arm that pushes on the plunger. If its smokey under mid to high rpm and doesn't quickly clear when you accelerate then reduce the size of the main jets. Check your timing, you might be able to increase the timing advance to allow more time to burn more of the fuel and increase power and economy. Also check that the disy is in good nick and the advance mechanism isn't flogged out. Check the vacuum advance is working. Worn shaft bushes can also effect spark timing between cylinders. And lastly check you don't have any vacuum leaks. Holden V8s commonly have leaks around the inlet manifold and hoses.

Cheers Lenny.