1977 Other Chevrolet Models Start problem warm engine

Hi,

My problem is following: Chevy El Camino. Cold engine starts fine, once it's warm it wont start anymore and I have to wait at least a couple of hours before it starts again.
Automatic choke closes and opens, I already checked that.
What could be the problem?

Car got a new engine a year ago and did not give me this problem till last week.
Carburator etc. Were not replaced because it all worked fine.

thanks to advise / William