Okay, I've had the same problem twice on the road, so I figured it would be for the best to put this in front of the experts.

I recently replaced the worn out carb on my 350 (purely mechanical, no computer) with a new Holley 650 DP. It drove well the 30 miles home until the last 2 miles when it stalled out at a red light without my foot on the gas. It seemed to stutter and then died. It took me several minutes to crank it up again but I was able to get it home.

The second day the same thing happened but was unable to get the engine to turn over. I actually drained the battery enough so that I had to get a charge from Auto Zone. Also I noticed under the hood the battery cables were in poor shape so I replaced the wires and connections. After an hour passed and I replaced the battery I was able to restart the car. It idled where I normally like it (1000 in park, 900 in drive) and I was able to take it home.

When I got it home I let it set and it replicated the same conditions that made it stall out previously. If you keep your foot on the gas it'll stay alive just fine, but let it set and the RPMs gradually fall until it stalls. Once it stalls you won't be able to restart it for awhile.

Any advice? It seems to idle normally until it starts to loose steam and gradually decline in RPMs until stall. When I get it home from the carb change I noticed black soot on the concrete from my downward firing exhaust, not seeing it now but then again it doesnt idle long enough
to leave the residue..

 

I'd check the fuel pressure. Sounds like it's too high, and it's forcing fuel past the needle and slowly flooding the motor. Check your float level too.