Driving it off the cliff

[blue62]

Originally Posted by ike84

It's 5 inches of water.

Most modern engines work on positive crank ventilation (pcv) in order to evacuate blow by fumes and funnel them back into the intake. As blue said, ours actually works in opposite to achieve the same effect, plus to seal the low pressure rings (although there are reports of running our crankcases just vented to atmosphere, which transfers them to a PCV system. But I digress ...)

This is a tiny vacuum. The cap should come off easily but you can feel a pull if you put your hand on it.

Slack manometer is the tool for measurement. This is one way to actually tell if your AOS is bad (AOS failure allows more vacuum pull of the crankcase)

The idle changes because the intake pulls on the aos, which pulls on the crank case, which communicates with the fill tube. Open the cap and you've essentially created a vacuum leak past the TB, letting in unmetered air and causing the idle to surge.

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5 inches of water
That's on the max end I believe
Do you know what the low end spec. is???
just so I can log the info.