I tried this on my daily driver Caprice V8 305. I hooked up a vacuum guage to the manifold behind the carb (routed a hose under the hood into the window to guage), and drove the car to work. Vacuum seemed to vary normally, high at coast around 23 Hg, down to 15 to 20 at cruise throttle, and down to zero at open throttle. That seems OK, but what I am wondering about is the vibration in the needle at part to full throttle. The needle vibrates about one half an Hg, back and forth a quarter Hg in each direction.
It vibrates much less at light throttle, and not at all at coast.


What does it mean ?

Thanks

 

Wow, my 305TPI readings are almost identical to yours but I don't have this fluctuation though. What kinda guage are you using? I've been using autometer for 3 years now and it works very smooth. Maybe a vac leak is causing your 'glitch'

 

I am using a MightyVac vacuum pump which doubles as a guage. I think the guage is working right. I think the vibration is coming from the vacuum vibrating due to something mechanical happenning in the engine. The mechanical cause of the vacuum vibration is what I am trying to figure out.

 

I get pulsing in my vac gauge if I connect it to the manifold. It is particularly pronounced on the Performer RPM manifold. If I connect the gauge to the manifold vacuum port on my carb all is sweet.

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Cosmik Debris