I just bought a 69 DeVille with a 472. My problem is rough idle. The engine already has new wires, electronic ignition, new plugs and pcv. I have replaced the carb, put new vacuum line for the brake booster, and adjusted the timing. The dwell is 34 degrees, timing 5 degrees, idle speed 550 rpm. The exhaust sounds good with no miss firing "puffs". Fuel mix adjustment seems to have no effect. My plan is compression check next. I have looked at all of the vacuum lines at the carb and no leaks there. Off idle, it runs great. The former owner put a flowmaster clone muffler which was way to loud. I added a resonator which helped make it a little quieter. Any suggestions?
Love this car. Also have a 74 Eldroado. Drives very smooth but I like the DeVilles performance.

It does kinda seem like a vacuum leak could be the cause. Have you tried taking a propane torch (or something that emits flammable gas) and, with the flame off but the propane flowing, aiming it at the vacuum lines and connections? With the idea that a vacuum leak will inhale the propane and the idle would smooth out.

It does kinda seem like a vacuum leak could be the cause. Have you tried taking a propane torch (or something that emits flammable gas) and, with the flame off but the propane flowing, aiming it at the vacuum lines and connections? With the idea that a vacuum leak will inhale the propane and the idle would smooth out.

Hmmm that may just work, however wherever the vaccuum lines are going... well there will be no vacuum. Just keep it away from the distributor!

Run the compression test first though... its best to get all of the known data out of the way before we start ripping apart stuff!