Hey guys,
Just wondering if I'm on the right track here. I have a 68 Valiant with 3.23s a 727, and about a 2400 stall. The motor is a crate 360 so I have 230/234 duration on a roller cam with a M1 single plane intake. The ignition is all MSD with a Digital 6 with a Pro-Billet Distributer. For a carb I bought a 750 Street Demon with Vaccum Secondaries. I have a stumble off idle and it seems hard to start. Does my set up sound right? Do I have to change something or just play around with the carb more? I have worked a bit with cars in the past but have never really done any tuning. So any suggestions or tips would be appreciated.

What is your initial timing set at? Being that your carb is a vacuum secondar, it's going to be a little hesitant when you punch it, but im gonna guess your timing is off since you said its hard to start. I would personally start at 12 to 15 initial, but there's some others on here that may have more knowledge about where to start.

A proper working vac. sec. carb will not stumble any more than a double pumper...sometimes less. We have raced in the past with success using a modified 780. I would check all the basics, timing, advance, vacuum leaks, etc. before tinkering with the carb, especially if it is new. If you suspect the carb, maybe you could borrow a buddie's carb to see if that fixes it, and go from there?

New does not necessarily mean properly adjusted, nor ideal for your combination out-of-the-box.
Could be jet size
Could be accelerator pump adjustment
Could be primary shooter size or accelerator pump cam - shooter too small or not enough pump shot, or pump shot ends too soon
As previously said, I'd also look at the timing.
Vacuum secondaries only have bog problems if not properly set up; that's what the diaphragm spring kits and secondary shooters are for.