70 2.0 webers - engine racing at start
 

I am bring a car back to running condition after sitting for ~20 years. I've completely replaced all the fuel system, carbs are rebuilt, and have had the car idling and trying to get the carbs set up well enough to run around the block. I've been reading up and have started it up once or twice a day to burn off oil and try to adjust the carbs. This past weekend I ended up adjusting the idle mixture out to 3 1/2 turns and the car really responded well. I shut it off and put it away for the day. Yesterday I went out to give it another shot and realized I had run it out of gas the day I went to 3 1/2 turns. Put more gas in and now when I fire it up it revs like its floored until I turn the ignition off. Any idea how this would happen with the butterfly valves closed? I'm 99% sure it didn't develop a huge vacuum leak in the past week. Any ideas would be appreciated. Cleaned out the idle jets - same before and after cleaning. Fuel pressure is 4 lbs - don't see anything different from previous days when it would idle fine.