You may have to do some fine tuning of the carb adjustments for idle mixture, main jets or idle speed settings and check for the proper power valve as well based on engine manifold vacuum at idle. If the power valve is wrong, it can result in an overlean/stalling condition under load such as idling in gear. Most carbs out of the box need some adjusting or swapping parts based on engine/cam specs. Make sure the distributor advance is plugged into the proper vacuum port on the carb as well...some carb ports have ported vacuum and some have manifold vacuum which can lead to timing issues with a wrong vacuum source...ported and manifold vacuum occur at differing times.

Demon carb tuning ...or call the Demon tech support hotline..

http://www.demoncarbs.com/Assets/docs/Pr...

"The power valve, located in the primary metering block between the two main jets, offers a boost in

available fuel to the engine in high load conditions without creating an over-rich idle or part throttle.

Power valves are marked with a vacuum rating. This rating is the point at which the valve will open.

The lower the number, the more the vacuum has to drop for the valve to open. When the engine is

under a big load, it produces decreased manifold vacuum along with an increased demand for fuel.

When the manifold vacuum is less than the rating on the power valve, it opens and allows extra fuel

to enter the main circuit of the metering block. It is a good idea to have a power valve rating that is

lower in number than the vacuum produced by the engine at idle."

When tuning Holley carbs and power valves, I like to use one that is rated for 1" less vacuum than vacuum at idle for good off-idle fuel transfer. Buy or rent a vacuum gauge and see what your manifold vacuum is at proper idle and remove the power valve and see what it's rated at...replace as needed.May have to tinker with a couple various ratings to get the right one....especially with high lift cams and rough idle/non-constant vacuum readings. Set the idle speed as best you can before taking vacuum readings.

http://www.corvetteforum.com/forums/c3-t...

My hot rod does the same thing. It's because it has a very healthy high performance cam.

Does YOUR engine also have a cam with rough idle?

You could also check that the idle mixture adjusting screws are set properly.

It sounds like the idle may be too slow or your have the idle mixture adjustments turned in too far and are running it too lean.

You have a vacuum problem. It could be; leaking intake manifold gasket or carb base gasket, vacuum advance to the distributor not on, power brake booster leak, open port on the intake manifold.

loaded idle is set too low.

I'd say the idle jets are too small.

i have a 1972 chevelle with a big block merlin 509, and demon 850 carb. In park and neutral the car will idle and run fine. But once i put it in gear it slowly dies. i have to stay on the gas and brake to keep it running at stop lights or stop signs. what could he the reason for this?