My '69 'Bird has developed a rough/low idle. I adjusted the carb and it runs fine in the driveway, but after a test drive it won't hold the setting. The car has been upgraded with the Pertronix and a Holley carb. Could the filter be clogged enough to cause a low/rough idle, but run smooth while driving?

could be a small vacuum leak that shows up after the engine is warmed up.

429 I assume. Did you just install the Carb? Spray starting fluid at the base of the carb to check for leaks. Spray the vacuum lines and PVC. Look for a increase in the idle - tells you where the vacuum leak might be.

I've sprayed all over, can't find any vacuum leaks. carb is been on for several years now. I replaced the plugs, several of them were so worn, I don't know how it ran. The new plugs are gapped at .035. I'll look at the PCV and see if it's bad. I'm not sure how old that is.