carburated dodge truck won't idle

hey guys, recently I bought a "new" caburator for my restored 79 dodge 1/2 ton. its a carter 2 bbl and looks new as the guy suggested it was so I paid $50 for it. at idle it won't run for anymore then 5-10 seconds and dies out but on fast idle it works fine, I tried another carb on it and it works fine as it should. what should I look for when I take the new carb apart? what would be causing only the idle circuit to not funtion? any advise is appericated its a 318 CI by the way

carter 2bbl.jpg