Weird vacuum behaviour?

[Soundy]

Been meaning to ask about this for a while...

When I was checking the ignition timing on my A20A2, I pulled off the #2 vacuum advance hose as directed... the engine started sputtering and slowed to a crawl, as expected, and plugging the hose smoothed it out and brought the idle back up, though not to the same level it had been.

HOWEVER... simply pinching off the hose with it still attached, which SHOULD have the same effect, instead has NO effect.

With the hose attached, pinched or unpinched, the timing is at the 20-degree mark at about an 800rpm idle... with the hose disconnected and plugged, it drops to 650-700 and the timing mark is nowhere to be found, even with the distributor twisted to full-advance.

The real question is, shouldn't pinching the hose off while still attached (and I mean, pinching it TIGHTLY with pliers) have the same overall effect as removing and plugging it???

[A20A1]

Well not really. Since vacuum is still trapped inside the advance diaphragm. Between the pliers and the distributor.

If vacuum can't bleed off some how then pinching the hose wont have any effect.

You can get a vacuum gauge hook up a "T" fitting for the gauge to tap into. Plase the "T" inbetween the distributor and the place you're going to pinch off.

Check the gauge reading before and after you pinch.

[Soundy]

Oooooooooof course.... d-oh! Makes perfect sense, thanks