low idling after a cold start?

I have a 1988 Nissan Stanza. It's carb-fed and here's my problem...

A month ago when I started the car early in the morning, the engine keeps dying unless I held my a foot on the pedal and keep it at 1000rpm. Initially, the RPM reads 300 or below. It's not until a minute when can let it idle alone without dying.

I already changed the fuel filter, had the carburator serviced for cleaning, and checked all lines for leakages (there was none). What could be the problem?

Thanks!