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.

[Ride The Snake!]

Originally Posted by 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.

I have only had the vac canister connected to the ported vac source on the carb, which wouldn't have any effect at idle anyway. My suspicion that it was blown was that I didn't see any discernible difference in advance movement while revving the motor and shining my timing light, plugged in vs disconnected. But that's a good suggestion; I'll hook it to the manifold source and see if it affects the idle.

[BigBird05]

Do you have a hand vac pump. Hook it up and look at the vac advance to see if it moves and holds or does it leak down.