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Is this a "normal" vaccum line ?

I was trying to use it to read manifold with a GM MAP sensor and the results are a little odd.

With the ignition on and the car off (so power to sensor and engine not running) it reads 5 V, which matches for atmosphere.

With the car idling, it STILL reads 5 V, or just a shade lower - atmospheric pressure at the distributor.

When I rev the engine (which should bring the manifold pressure UP from vaccum), the voltage goes DOWN, finally indicating vaccum.

This is making NO sense.

thats correct for the advance vacuum i think, as rpm increases so does the advance........
I think for what you want to do is tap into the manifold...might need to drill a port, actually there should be one there, only not fully drilled through,

The dizzy will see whats called "ported vaccum" i.e it doesn`t see vaccum when the throttle is closed/@ idle...only after the throttle is cracked a certain amount/on cruise.
Not having vaccum advance @ idle increases idle stability and torque for take off....you only want advance from the dizzy vaccum module to aid fuel economy on light load.

Drill a hole in the manifold...or maybe there is another line there you can use/T off.

OS

Ahh, OK. Didn't realise there was some smarts in the vacuum going to the distributor.

It's a carby EA 82 in an '88.

I'll have a look - I think there's a manifold vacuum line, but it's just further from the sensor.