My truck(78' with a 400) was missing a bit so I hooked up the vacuum gauge to the manifold tree and took a video. It was a warmer day and did not seem to miss much this time.The gauge needle shakes a bit. Can you look at the video and comment on the gauge readings? Thanks.
YouTube - Ford 78' 400 Engine Vacuum Test

 

This is interesting. I love the vacuum gauge and using them can actually tell you more about an engine than meets the eye.
Where do you have the gauge connected to? It is right on the manifold correct? Not on the baseplate, but directly on the manifold?
I have seen variations when manifold vacuum is not used, or the manifold vacuum is tapped in from the baseplate. Should be the same, but some times it is not.

Most of what you have there looks pretty good, but you do have some fluctuation.
Not a bunch but some, and to me this might indicate a valve that is not sealing properly. Either a weak spring, bad valve guide or a valve seat that is not well.

A compression test as well as a leakdown test might be interesting to see.

You may also look at the PCV valve, and make sure that is is clean and operating correctly. WHile this pcv problem might make the needle have more movement, this is still a possibility.

Small, misfire can also create this condition, so be sure that the electrical is up to par. Even if the parts are new, be sure that everything is good and there are no leaks in the spark plug wires are anything.

Still leaning towards a leaky valve though..........Subscribed to thread.

 

Looked perfect to me.
Looked like a steady reading of 21, 22 in.
That's good strong vacuum.
I think you'll always get a LITTLE shake of the needle.
I didn't hear it missing either.
If it was, maybe it was lost in the audio?

 

I connected it to the tree that comes off of the intake manifold directly behind the carb. The day I shot this it was not missing nearly as bad as it has been. The only difference is the temp. outside was 10-15 deg. hotter.

 

I think if the problem is one valve, the vac. needle will drop
noticeably (5 in or so) only when that valve is supposed to seal.
So, the needle would drop roughly a couple of times every second or so.

 

Getting a compression tester soon. I will post those results.

 

I cant argue against this Murph, but I can add that the OP did state a few things that can support at least part of my theory. Perhaps if the valve guides were worn, the leak is actually not at the valve seat but through the valve guides. Perhaps we may ask if the truck puffs blus smoke?
That being said, the amount of oil that it consumes might explain the slight miss, and the related temps when it happens.
This might also support the relationship between when this happens and not only the outside temps but the engine temps as well.

All of this is just speculation Murph, but I do like the conversation. Your point is quite valid.....

 

Yea, I was just re-thinking this.
We really need to see another vac reading when the truck is acting up.
I think a sticking valve could be an intermittent problem... if the problem isn't too bad yet.

And Berry, if funds are keeping you from getting a compression gauge,
I'm pretty sure parts stores loan or rent them.
Those numbers could be pretty important to have, as mentioned above.

Murph.

 

I will be getting a gauge shorty. Also, no blue/white smoke at all.

 

Looked real good to me.

How new are your spark plugs? If they are brand new it isn't too uncommon to have a bad one in the bunch. That could easily cause some problems with cooler air and missing versus warmer weather and smoother running.

Josh

 

The plugs have about 1,000 miles on them.

 

Looked like a pretty rock steady reading of 21" to me (other then when you were working the throttle of course)

 

Here is a really good tutorial on vacuum gauge readings.

How to Use and Interpret a Vacuum Gauge

 

On my new 78, the PO installed a vacume gage in the cab, nice tutorial. Not to sure where he connected it on the eng, but will investigate when I can.

 

This is another useful site.

Link: On Fire

This takes you to the vacuum interpretation part of the page.
Look around the rest of the site for more good reading.

Murph.