1981 Fiat Starting Problem

[So Cal Mark]

a large vacuum leak will prevent the engine from starting. That brake booster hose is 1/2" diameter and can cause a giant leak. Didn't you mention that the engine starts for a second, then stalls? With the key on, engine off, open the AFM flap and you should hear the pump run

[rlux4]

Mark, I gotta pick your brain again. I'm trying to understand how a vacuum leak would prevent an engine from starting. Is it because it robs the necessary suction to bring fuel/air into the cylinder, or does it cause a lack of sufficient compression, or something else I'm not thinking of?
Ron

[So Cal Mark]

the mixture is too lean to fire

[rlux4]

Thanks Mark. Let's see if I have it right. It's allowing too much unmetered air into the mix?
Ron