IMO you have some problems. I am uncertain how deep your problems are at present being I question how accurate your vacuum gauge is (US General Harbor Freight). Not saying it is a junk gauge but I would love to validate it before assuming some things. What I am certain of though is that the rapid vibration of the gauge when in gear generally indicates a sealing issue with a valve train component whether it be a valve or valve guide issue. I see some float at regular idle which is obviously higher RPM. That is likely a carb mixture issue which I'd overlook that for now. The rapid bouncing has me pretty convinced you have a valvetrain issue either with a valve guide or valve seat issue. If that's indeed the case that means pulling the head and if you pull one and refresh it you might as well pull both.

Course of action: You want to know for sure you have a sealing issue. I would run a leak down for sure on that cylinder. Something isn't right, and I expect you'll find something hissing through the exhaust or into the intake. Could be a valve guide which a leak down may not show unless you force the valves open on the cylinder a few times seeing if it takes a different seat. The fact that it's only showing 14 Hg in of vacuum has me alittle concerned. Should be close to 20 particularly if it's a stock motor, but again I also question the accuracy of the gauge. If the gauge is legit then you want to leak down the whole motor to see what kinda shape she's in. Could just be stretched timing chain (I'm assuming the motor is stock and still has the OE timing set). You can check that by pulling the dizzy cap, pulling the plugs, and putting a bar on the crank. Roll the crank to 0 on the timing indicator (can be 180 out). Roll the motor until you see the dizzy rotor move and make note where on the balancer. Then roll it back the other way until the rotor starts to move. Shouldn't have more than 5 degrees of variation if it's good and tight.

I would see what others thing but that's what I'm reading from it.

 

Sorry about the back issues. I totally understand given I've been under the knife to relieve some of the issues I was having. All I can say about that is don't let yourself suffer like I did. Eventually one of my discs was so badly herniated that it was actually pinching my spinal cord causing terrible spasms etc. What was supposed to be a simple microdiscectomy but the doc realized that the MRI didn't show how extensive the issue was and ended up having to remove the entire lamina from my vertebrae to fully clear the issue. Went from a 1 - 1.5 hour outpatient procedure to a 6 hour surgery with a 2x night stay in the hospital. With that said I'd do it again in a heartbeat. In my case I had terrible shooting pains (sciatica) down my right leg regardless of how I layed, sat, stood, and so it. It was a 24x7 hell. That and the spasms. The spasms were what I consider the equivalent of getting nailed by a 220v power circuit. Hurt like hell and would just reduce me to tears. No drugs could stave off the kinda pain I was having and leave me actually functional. The pain from the sciatica and spasms was so terrible that suicide was a very real consideration given how bad it hurt. I was in my late 30's at the time and no doc even wanted to consider looking at me for surgery because of my age. When I finally found the doc that did my surgery he took one look at my MRIs and I was in the OR in 3 days, that's how bad it was and the true extent wasn't seen until he was in there. So if your dealing with something like that don't let yourself suffer and see a surgeon. Best thing I ever did.

Back to the leak down test and engines. What do you think you were doing wrong? Leak down tests are pretty straight forward unless you have a tool issue.

 

If you take that vacuum fitting off and just plug the hole do you still have the miss in #8?

 

Have you tried new plugs? In my experience when plugs fail they can do so sporadically. Plugs are relatively cheap and easy to change and some of the suggestions here not so much. The fact that you swapped plugs doesn’t convince me. If you put new plugs in it and the symptoms are exactly the same, then I’d be convinced.

 

It could have a cam from the previous owner but I'm assuming stock, so yeah vacuum should be higher. Intake is aftermarket. I'll try the other tests, but I've suspected that at the very least the valve stem seals were toasted for awhile. The thing still easily passed smog check a few months ago which is usually a good health indicator but I could of broke it doing some vette things since then. If that did it well it was still fun.
For the leak down first try things read well, low leak down percentage, but neighbors were about and distracting me so I stopped for a bit, then went and started over when they were gone, this time very high percentage. So could be me or the tester (HF again). The important take away is in both test runs when I cranked up the air pressure I didn't hear anything from the intake or exhaust, only crankcase. To double check myself I rotated the motor to where valves would be cracked open just to make sure I would be able to hear the air escape from intake or exhaust, which I could in those cases. I will probably try again with all 8 cylinders full blown test.


Have not tried that, but I can, even if its just using my thumb, so will give it a go.

Not yet but if I am gonna pull them all for a leak down test I might as well change them all with new ones, at the same time I may pick up a better vacuum gauge.

I've been unemployed for awhile so this will be a budget job, I have some time on my side tho since its not due for a smog until Aug 2022. I also want to get the camera in the cylinder, if the short block is ok that would be a huge thing and I could look at this as an opportunity for a head swap.

Final question, is it me or does that intake runner look way too oily? Which would again point to worn heads

 

Figured I would take this separately.
Sorry to hear about your back probs, mine started when I was around 30 yrs old, I'm 50 now. At the time they could not find any disc issues and said it was all muscular, don't know if that's changed but a common theme is whenever it acts up my back gets a little crooked, but sideways, not hunched over.
I am in a better place with it now than 20 yrs ago. I had it pretty well managed with morning yoga and a good exercise regimen. I had a job with a company gym and a nice nearby park to walk a couple of miles every day. Then I changed jobs and lost those, but at the new job we played ping pong every night which surprisingly was a helper.
Then I was laid off, but undeterred I got a gym membership and went 3 times a week.
Then COVID, so I've slipped on doing what I need to do to keep the back in check. I could do more yoga and use my treadmill, just need to get my act together.

 

Hey shoc I pm'd you last night. Look at the far right corner of your screen next to your login name.

 

Doubtful it has a cam that would lower the vacuum that much, you would know it based on the idle behavior. Think chop chop kinda idle. There will always be some blow by into the crankcase due to how engines are built so I wouldn’t get too excited about it. Considerable leakage quite frankly you would notice on a plain old compression test.

I would redo the leak down and work on getting repeatable results. Harbor Freight has two different leak down tools and from what I’ve read one of them has an issue with consistency.

You really need to be putting around 100 psi of pressure into a cylinder with it at TDC (you’ll need a strong buddy and a long breaker bar to hold it) to know what’s really going on. Anything less than that and it’s really hard to tell.

Eric O from south main auto gives a pretty good how to and the basics on it.

 

Will go look.

First thanks for all the suggestions.

Ok so its fixed but I can't really say why, which is frustrating. I can tell you what I did and we can guess from there.

I removed the vacuum nipple on the #8 intake runner again and with a long tube sprayed carb cleaner in wile the engine was running.

First few sprays didn't really do anything, sprayed some more and then the rpms started to rise.

So then it was a bit of a juggling act but while spraying I would pull the #8 wire and it would drop engine speed, after that even without the carb cleaner acting as extra fuel I could pull the wire to #8 and it would now affect idle, not as much as say pulling a wire from the other cylinders, but still it had an obvious affect where before it had none.



So then I just ran a ton of cleaner and things improved more to where the idle drop test shows 8 is doing its fair share now. Ran the camera in the runner and the cylinder and was visibly cleaner but no surprise there.



Since the valves were clearly moving all along is it possible there was an issue with the piston rings being stuck or something?



Either way while the main issue is fixed the tests along the way have indicated that I probably need the heads rebuilt and a full leak down test at some point would be interesting.

 

So the assumption is there was quite a bit of build up on the back of the intake valve acting as a sponge for fuel or by spraying cleaner created a seal on a defective gasket? Carbon piece stuck on the seat or valve washed off? Good you got it fixed. Odd issues can be very frustrating for sure.

 

Glad it seems you've sorted it out. Curious what the vacuum looks like now.

 

Hope that turns out to be a permanent fix. I still like a leak-down test to be sure.