My 72 250 has what sounds like a vacuum leak, however I have tried everything and I can pretty much guarantee that its not a vacuum leak. I've plugged every carb and intake port, changed carb base gaskets, everything. I did find out I had a bad vac. modulator on the trans, and I replaced that but haven't run it yet. It doesn't seem to affect the way it runs, and if I open up a new leak (by pulling the brake booster line), it stumbles, like it would when there's a vacuum leak.

What could it be?:homer:

I've found gasket leaks on the manifold and carb flange by spraying brake clean on the gasket and when it changes the idle you've found it. You can try that.