'50 Cadillac Runs Rough at Idle in Gear

[Bonnevillains]

Have a 1950 Cadillac w/ a 4GC Carburetor (installed ages ago, recently rebuilt.) Runs rough when idling in gear, the engine shutters but continues to run. Restricting air (hand over carb, or closing choke) eliminates shutter. Air cleaner installed makes no difference. Runs great when idling in park/neutral. Runs great when accelerating and cruising. Idle mixture screws set correct. Fast and slow idle screws set correct. Timing, points, dwell, plug gaps all correct. fuel pump works. Jets all original from when it ran good. Only thing that was changed was carb rebuild (because it was dirty.) Any ideas?

[rocketraider]

Vacuum leak. 4GC carbs have several different fuel bowl-to-throttle body gasket configurations that can cause weird idle conditions if the wrong one is installed. Carbking can help you here once he sees this post.

[carbdoc]

Bonnevillains,

Glenn is right: you probably do have a vacuum leak somewhere, but please be aware that your carburetor was not installed by the factory. To my knowledge, the first installation of a 4-barrel carb on a GM product was 1952. There is nothing wrong with installing this set-up on your car as long as it was done right. And do have someone knowledgable check to be certain that the correct gasket was indeed used between the carburetor and the bowl as suggested.

Jeff Dreibus