Vacuum Gauge questions, help appreciated

[jocko]

Gents,
In the process of restoring an old pontiac vac gauge to stuff it in my 66. Figured it'd be a good idea to make sure it works first... When I bought it at the swap meet, I brought a vac hose with me, sucked on it when attached and the needle moved off the peg - so assumed it was good.

Now, when I hook it up to my engine, same port where the vacuum advance is normally hooked up, I get the following:

At idle - 0 in Hg
As idle increases, needle comes off the peg steadily and at about 2000 rpm or so, it's at about 1 in Hg.

Considering the engine has no load on it, wondering if this is about right?

I expect it to decrease again if I gun the engine, and it kinda does blip back down toward the power end (drops from 16 to about 12 in Hg).

Bottom line, it never gets over into the "economy" end of the gauge except at the very low end 16 in Hg max - never sees 17-24 - the full green economy end of the scale.

Anyway, this seems "about" right to me - I'd like it to have a little more vacuum, but it might just be off a little bit too - and that's ok.

But my question is this, do the numbers and directions and amounts and reactions to quick blip on the throttle make sense to you? At first it seemed odd that vac would increase as rpm increased (i.e. adding more "power" - the 0 end of the gauge...) but then again, of course, vacuum increases with rpm. Also makes sense that it temporarily drops when blipping the gas a bit. I expected more change in the needle when I pulsed the gas pretty hard - but it only dropped 3-4 in Hg then right back up to 15-16 in Hg. Just seemed a bit odd, wanted to confirm - I believe it's probably about right.

Thoughts?

[billd5string]

On my Edelbrock carb, one of the ports on the front is documented as "timed vacuum" and it acts like you describe, no vacuum at idle. The previous owner had that hooked up as the vacuum advance on the distributor and since other work that had been done on the truck was pretty messed up, I’m reviewing everything. I found the other port on the front of the carb acts as one would expect (plenty of vacuum at idle) and according to the docs I found online at Edelbrock, that is the “manifold vacuum” port for non-emissions controlled engines. The truck runs MUCH better now using the manifold vacuum port.
Maybe you have a similar situation, using a timed vacuum port instead of manifold vacuum.

[jocko]

ha! double checked withb a known good vac gauge i forgot i had. reacted approx the same way. For SOME reason, I had in the back of my mind that there should be about 16-18 in Hg at idle in order to operate power brakes - but I think i got caught in the old ported vs non-ported thing again.

Where should a vac gauge be hooked into - what circuit - for the proper readings? My carb doesn't exactly have vac ports protruding all over the place, nor does the manifold. Thanks for your help.

[jocko]

oops - simo posted bild - thanks for your input. I only have one port on front, old carb and the vac adv is hooked to it - but the truck runs fine as is, so that is correct. Just curious where I should hook the vac gauge.

Are you saying you hooked your distro vac advance to the full time port that shows 16 in Hg at idle?? That doesn't sound right to me. Or are you saying you hooked your vacuum gauge to the one that shows 16" in Hg at idle?

[markeb01]

My vacuum gauge is connected to full manifold vacuum and reads 16-18" at idle. Descending a hill with the throttle closed and the transmission in gear will generate 20-22" of vacuum. Standard acceleration drops the needle to 6-10 inches, and wide open throttle will take it to zero. I’m running a 350 with a stock cam and about 10-12 degrees BTDC initial timing. These are pretty much the numbers I’ve come to expect ever since I started driving.

I have also had several vintage Stewart Warner and Sun vaccum gauges that were 30-50 years old, and every one of them read low. Some were 2-3 lbs low while others were off by 10 lbs or more. A couple of years ago I checked with every one of the custom gauge rebuilders advertising in magazines and on the web. At the time none of them restored vacuum gauges, so I gave up on all of them and now run a new production Autometer gauge.

If your power brake connection is the only full manifold source available, you could insert one of the nylon/plastic adapters in the power brake hose to feed the vacuum gauge. They come in a variety of styles and hose sizes.

Another tip for checking the accuracy of a vacuum gauge is to hook it up to a Mityvac hand vacuum pump if you have one for bleeding your brakes. Just compare the reading on the pump to the one on the gauge. On mine the pump and the gauge both matched so I knew I was okay.

[jocko]

thanks markeb01, i did confirm with a handheld vac gauge - the truck gauge I'm gonna install was only reading an inch or two low at the most, so it will be good enough. However, I do need to find a full time vac ource to tap into - don't have pbrakes (yet) was only mentioning as I thought I SHOULD have 16+ in Hg at idlel, that's all - that's why I thought my readings were off. Thanks for confirming I need a full time source - the vac advance is part time I believe.

Once I get the gauge installed, etc, I'll find a vac source, that shouldn't be too awful hard - planning to upgrade to pbrakes soon, so need to find one anyway. And you're right, it might take a special t-fitting (i.e. large pass thru with a small t-nipple on it for the vac port). Again, thanks much for the help - you answered my biggest question. I've confirmed the gauge works, and just need to find a full time source.

I forgot what a pain in the butt yankin the dash is on this thing....
Have the gauge lines and needle painted at this point, gotta crack open the dash to insert it all, unfortunately can't just slip it in from the back because I'm inserting the whole cup with the gauge for proper green-lighting - that was a whole "nother" issue....

Will post up some pics, if/when I ever get it all together. Stalled at the moment while paint dries! Bought this gauge at Carlisle years ago, just now getting around to it.