I have a brand new 850 demon carb on a pretty hot 406. My problem is idle is fine or wide open is fine, but if you gradually accelerate it will choke completely down. I have checked the site bowls and both are right at the bottom. Any obivious things to check, timinig looks ok plugs ok. Was running fine with a holley 750.

I have checked the site bowls and both are right at the bottom.

Float levels should be set at half way in the site glass.

Technically that carb shouldn`t be on that motor.
BG says it belongs on 470-500" engines.
The problem is in the transition from the idle circuit to the main circuit. Not enough air speed to activate the booster smoothly.

You might need to reduce the idle air bleeds, increase the pump shot, both in volume and duration via squirters, pumps, cams etc..

You also need alot of initial timing as much as 20-22 degrees, you`ll need to limit total though.

With some other adjustments you MAY get that dead spot out, other wise you`ll need to move to a smaller carb.

If youre talkin about it wanting to nose over when coming off a stop sign, then its the idle feed restriction - you'll need to open it up a bit. Its common problem on demons.