Instrument cluster vacuum meter reaction time?

[jcwren]

My '67 Barracuda has the vacuum meter option. It's not currently connected to the engine because all the tubing is shot. However, while I had the dash out to replace the instrument voltage regulator, I tested the vacuum meter with a hand operated pump. The needle moves, but verrrry slowly, much slower than would seem to be useful for when you put your foot into it and want to see it drop into the "performance" range (as if the original /6 really had a "performance" range, but I digress).

I had an idea of how it should operate, but the gear reduction it's impossible to really see any physical change in the what I'll call the bellow. When I pull 25lbs vacuum as indicated on the pump, the meter slowly creeps up to 10. Maybe if I keep holding vacuum long enough, it would go higher, but my hand is cramped by this point. When I release the vacuum it takes nearly a full minute to drop back down to 0.

How fast should I expect the meter react?

 

[67Dart273]

............A lot faster than that. If you can get to the "works" I'd give it a shot of brakecleen and or maybe a bit of WD or better yet, likely spray silicon lube

Also, many vacuum gauges had a restriction/ orifice to keep from showing too much pulsing. Possible it is restricted / gummed up, etc

 

[jcwren]

Cool. I'll try that and see what happens. Thanks!

 

[mvh]

Mine is super responsive to every little movement of the gas pedal. Sounds like yours is stuck.

 

[qkcuda]

The one I had needed the inlet opened up with a small drill, and after that it worked fine. It was almost like it was filled with solder.

 

[67Dart273]

Also, no idea how large the inlet orifice is. Do you have a set of acetylene torch tip cleaners? Might see if you can gently get one of those in there

 

[qkcuda]

Yes, best to sneak up on it slowly, and test it with your vacuum pump as you go.

 

[Lord Sparky]

All great suggestions. Just adding maybe a good Speedo shop can fix it if you can’t. They tested/tuned up all my meters.

 

[jcwren]

I don't see anything that resembles an orifice. It's got what looks like a pot metal plug in it. I've looked at with various angles under the microscope, and I'm not seeing how air is supposed to bleed past the plug. The plug can't be removed because it's been peened in. I'm not adverse to putting a hole in the plug, but I am concerned about keeping any debris from drilling it out of the bellow.

 

[4spdragtop]

Should be a tit/nipple on the back...not sure what you posted a pic of?

 

[67Dart273]

It may be some sort of filter similar to the sintered metal filters used in Holley/ Quadrajet carb inlets and others. Other than that, I don't know. If so, that filter could be plugged / gummed

 

[jcwren]

Should be a tit/nipple on the back...not sure what you posted a pic of?

You're correct, I didn't show it. It's just a nut with a nipple on it. There's no filter, metering orifice, or anything like that. And it's not blocked.

 

[67Dart273]

I cannot tell what it is. Maybe it's a piston?

 

[jcwren]

It's not a piston. It's the inlet port of the vacuum gauge. Here's a different perspective. It's not sintered material in there (I know the filters in carbs that you're talking about), it's pot metal of some kind. Kind of soft, but harder than lead. I think I will try drilling a small hole in it, something like a #60 (0.040) and see what that does. Worst case, I end up drilling the entire thing out and putting a disc with a precision drilled hole to reduce the responsiveness.

The metal plug is staked on either side (I said peened earlier, that was wrong).

 

[jcwren]

I drilled a #70 (0.028") hole in the plug, and it's working now. I'll have to see how responsive it is to engine vacuum, but with the hand pump it's tracking right along with it. An added benefit is that it's matching the gauge on the pump, so it's >somewhere< near calibrated.

Thanks to everyone who provided guidance and suggestions.

 

[MopaR&D]

Kind of strange but I suppose the factory did that to reduce fluctuations from a less-than-perfect tune and/or fouled plugs, etc. My Auto Meter vacuum gauge has no "delay" at all, it'll flicker up and down as my engine idles if it's cold or I don't have the tune spot-on and it moves instantaneously with the throttle. I prefer it that way.