Will Not Idle

[mtuck]

I need some help. I am trying to get my car running and i am having trouble. I have a chevy 350 that i have rebuilt. The car will start up and run as long as i give it gas and keep it above 1500rpm. It is very rough running. But as soon as i take my foot off the gas pedal and the rpms come down the car dies. I can not keep it running. I can not keep it running long enough to set the timing. Any suggestions that i might look at? I have a new holley 750. I checked the floats and they seem alright. I also have noticed that while it is running, i am having some gas "pop" back up through the carburetor.

Any ideas i might try?

Thanks in advance for all your help
Matthew