is this diaphragm (connected to the secondary throttle plates) supposed to only work under condition orange? i noticed it wasnt doing anything while i was cleaning my BAC today, so during idle i unplugged the vacuum hose from it. no change. i hit the throttle. nothing different.

is this normal? (the part im talking about is in yellow)

 

When the thermo valve engages (normal engine operating temperature) it allows vacuum from the engine to suck that diaphram open through a one way check valve. The line should go something like this.

Diaphram>Check valve>Thermo Valve>Intake Manifold

Its designed so that you do not stress the engine when cold, it closes two of the three throttle plates and allows for only one bore in the TB to be used when cold. When the engine warms up, the diaphram should contract pulling open the secondary throttle plates, allowing for optimal air flow.

If the secondary plates are closed all the time, you will lose quite a bit of performance, but it will not affect idle.

You can test it by unplugging the vacuum tube from the thermovalve and sucking on it (heh..) the diaphram should open the plates and no air should leak out allowing the plates to shut again.

Hope that helps.