Strange Idle Problem

Stock '82 Plymough Gran Fury, 318/904 and Demon StreetDemon 1901 carb (Thermoquad clone). Car runs very well normally, but if ambient temp outside is more than 70*F, then after fully warming up it will idle at approx. 2-300 rpm low and rough, and eventually stall out unless I keep a foot on the brake and slightly open the throttle to keep the RPMs up. Vacuum gauge reads steady, and car accelerates with no stumbling or other issues. Any idea on the problem? Mixture and timing are adjusted fine, no apparent vacuum leaks (18" solid/steady at curb idle). Ignition is stock electronic ignition with a NAPA box, stock electronic ignition distributor is hooked up to ported vacuum.

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if your idle is dropping 2-300 RPM and becoming rough that would have to be reflected on the vacuum gauge.

Regardless, I would suspect a vacuum leak opening up, or puddling of the carb caused by excess fuel pressure / float levels to high

Make sure your throttle cable, if equipped, isn't too tight.
Also make sure all of your intake bolts are tight

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I would suggest trying the full vac port. or checking to see if the advance weights are gummed up/sticky and make sure the vac can is not ruptured and a vac leak when coming back down to curb idle.

mine did this with a bad vac mech advance can. had a slight vac leak made it go lean and stumble at idle.

I run mine on full vac and set A/F ratio with vac gauge with mech vac un hooked.

get my curb idle & initial timing set. hook up to full vac and it idles up and cleans up the idle. then readjust the curb idle back to the 650 rpm setting.

or I use the FBO mech advance plate/MrGasket light springs and set curb idle/initial timming with no vac advance hook up.

14* initial/mech advance plate on 18* for 32* total (no vac advance)

runs about the same way with either set up.

Wanted to give everyone an update: On closer inspection it turns out I was indeed plugged into manifold vacuum for the vacuum advance can on the distributor. Switched to ported vacuum and no more idle issues. Thanks for the advice!