I wish I'd read this before I did an engine swap. Let me warn those who do engine swaps... I've seen this happen another time since I did it on mine...
When you are lifting the engine (or dropping it, whichever...) in place, you might as well remove the anti-shudder valve before attempting to install the engine. What happens is the engine will rock back and hit the firewall, and guess what? The anti-shudder vavle is the first thing to go... the three plastic tabs that hold the vacuum pot in place snap off with extreme ease.
Unless you are looking right at it, you won't notice anything is wrong. Then, when you go to start the car, you get the dreaded start/die problem. Without the plastic fingers holding the vacuum pot in place, the air sucking by the anti-shudder valve closes the valve and kills the engine. Then, without the suction of the engine, the valve opens up, allowing just enough airflow to start the engine again.
But no fear, faithful readers. A zip tie locked the dashpot back into place and the engine fired off like a champ!