Falling on face/missing/backfiring (through intake)?

[dmjlambert]

I am curious what makes you suspect but not know vacuum advance diaphragm is blown? If you have the engine idling and you connect and disconnect the vacuum advance from a manifold vacuum source, you should notice quite a difference in idle speed. Put your finger over the vacuum port on the carburetor when you don't have the hose connected, so you don't have a vacuum leak. I didn't have that difference in idle speed on mine and that's how I knew the vacuum advance was bad, and I replaced it.