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Discussion Starter #1
On an Alfa Club tour today, one of the guy's series 3 spider crapped out. It would start and only run at high idle, then stall. I noticed the large main air duct on top of the motor had separated; I connected it and the car ran fine - please excuse the lack of proper terminology, I have a 74 without all the various hoses, etc.

Although the car sounded good and idled fine, one of the cloth-covered hoses would actually become flattened when we revved the car. Does this indicate a worn hose or is something else wrong?

Thanks very much!

John
 

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Discussion Starter #3
Here is a photo from a BAT Alfa ad. In the upper right-hand corner, there are two grey hoses. The one closest to the firewall was the one that was collapsing. I just wanted to know for my own knowledge and to give the guy a call. We do quite a few tours and a lot of members have series 3 spiders.
The large black piece that sits on the cam cover is the part that separated at the metal clamp.

Thank you!

John
 

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That helps. I'm still not sure I can explain it but I'll take a guess.

I assume you are referring to the hose that is attached to the item bolted to the cam cover. That item is the Auxiliary Air Valve (AAV) - intended to provide a bit of additional air when the engine is cold. The computer will then add the additional fuel needed by this extra air. The result is to increase the idle speed when cold. As the engine warms up the AAV closes and idle speed returns to normal.

Now then, here's my guess. As you revved the engine it was sucking in air through the large duct (the one you cleverly figured out had come loose). This causes a slight vacuum in the intake & air duct. If that collapsing hose was beyond its service life it wasn't able to stand up to that vacuum.
 
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