Vacuum gauge expert needed

[kcirtap1984]

Can someone please tell me what’s going on here?

Hooked up the vacuum gauge to the manifold. It’s pulling about 20.5 in/Hg at idle, which seems great to me. But the needle is vibrating +- .5 in/Hg. In my experience, a vibrating needle means that the valve train isn’t sealing. However it’s usually more of a +-. I would assume that it is valve guides or seals. But I just had this thing apart. The valve guides are nice and tight and the seals are new. The valves are seating well and the clearances are all within range. Is there something else I should be looking for? The engine setup info is below and there is a link to a video of the issue as well. Thanks in advance!


79 2.0 with a single plane and a DFEV. Fresh rebuild. New rings, rod bearings, valve seals, mild port and polish.


https://youtu.be/pYXpxBK6fBU (Open in new tab)

[18Fiatsandcounting]

My guess is that this is normal. At idle, say, 900 rpm, there are 1800 openings of an intake valve every minute (twice the rpm). This would be 30 intake valve openings per second. My calibrated eyeballs aren't good enough to tell if that's the vibration that you are showing in the video, but it looks like it might be. Each opening of an intake valve will cause a momentary drop in vacuum, which might be what you're seeing. If that's the case, you do have an extraordinarily responsive vacuum gauge!

Also check that your vacuum gauge isn't just lying on a part of the engine that is vibrating, causing the needle to fluctuate as it picks up that vibration. Try holding the gauge in your hand and see if it still vibrates.

-Bryan

[kcirtap1984]

Thanks for weighing in. The gauge does the same thing in my hand. I am going to drive a little more and then pull the plugs and look at each cylinder. If something is not sealing, I bet it will show up as heavy carbon deposit on the piston.

I'm not sure if the vacuum vibration is something to be concerned about, or if I'm just being paranoid. This was my first solo engine rebuild. I have been on "high alert" listening to every little sound it makes. I hope my anxiety goes away with time and miles.

[SteinOnkel]

Ahoy!

Is there a damper in the line? If not, the reading is pretty useless. The dampers are usually little brass pieces (they are tapered) that get shoved into the line. And they have a teensy wincy hole in them.

For my ITB car (where I need to measure each plenum individually) I bought the set in this picture many moons ago:

https://www.louis.de/rund-ums-motorrad/ ... ronisation

See those black knobs? They are the dampers and need to be constantly adjusted and tweaked while you are synchronizing your carbs, throttle bodies etc. Too much and the needle won't move it all, too little and it flies around all over the place.

[18Fiatsandcounting]

I'm not sure if the vacuum vibration is something to be concerned about, or if I'm just being paranoid. This was my first solo engine rebuild. I have been on "high alert" listening to every little sound it makes. I hope my anxiety goes away with time and miles.

Nah, you're just being paranoid. But not to worry, I get hypersensitive to every little nuance after I rebuild an engine or transmission, too. People think I'm paranoid, but that's just because they're out to get me.

I think your vacuum looks good. By the way, if the gauge needle periodically clicked downward, that could be a sign of a bad valve, but that's not the case here.

Steiny's advice above is good, too.

-Bryan