I just saw the video's. That is definitely internal and not an exhaust leak IMO.
First thing I would do is check the oil pressure next time you run this again.
Your vacuum gauge is telling me it's not exhaust and neither is the slight cylinder not firing on the throttle tip in. . IF you have oil pressure I'd be looking at a split rocker arm on the fulcrum providing the rest of the obvious stuff up top checks out.

Again just my $.02

Ok, I pulled the valve covers and inspected the rocker arms, pushrods, springs etc and did not find anything obviously wrong. Only some minor scoring/pitting on some of the rocker arm studs. I tried pushing down on each lifter to see if possibly they had a collapsed/broken spring and they all felt good tome. My next move is to pull the intake so I can pull the lifters to give them a better inspection, I also want to check the lifter bores to eliminate that. The attached pictures of the rocker arm and valve spring are representative of all of them.

That sound is definitely a clatter of sorts and it sounds like it has a small misfire. I'd definitely inspect the cam lobes, sounds like it may have a lobe going away.

You're going the right direction on your troubleshooting.

Well I just finished inspecting the lifters and cam lobes, I did not see anything obvious. I checked a few of the feet of the lifters with the rocking test, two lifters foot to foot and verify they rock a bit, and they checked out all right. All the lifter bores looked good and the lifters were tight in them and all the pushrods were straight. I've attached some pictures of the cam so if you see something amiss chime in. While I had the rocker arms all loosened I poured some sea-foam into the intake side of the head to test the valves and they held overnight.
With nothing obviously wrong I guess I'll just leave it as is, the only other thing I could imagine it would be is something in the heads but they appear to be good. I've got it all back together now and I'll wait till a break in the weather here in the PNW so I can pull it out and give her a tune up.

Why are the lifter footprints so far off of center ? It looks like the cam has walked to the back of the block...


https://cimg8.ibsrv.net/gimg/www.cor...6e0ef38a38.jpg

Look at the third lifter where it meets the cam lobe ......or the left (first) lifter angle.....

I noticed that most don't line up exactly as you would think, I figured its normal but I may be wrong. I don't know if the cam could move back, the distributor drops right in so those gears are lined up. The only other thing keeping the cam lined up is the timing gear as far as I know so those bolts would need to be loose for it to move. Something to keep in mind

The above lifter positioning is normal and is done to encourage lifter rotation and prevent wear. The quote below is from Crower.

" The offset between the lobes and lifters
aids lifter rotation and distributes valve spring
loading over a wide area of the lobes and lifters
assuring longer life. But most important, this
off-set design, combined with tapered lobes and
spherical lifter faces, is a specific cure for the
greatest cause of cam lobe failure-edge loading."

Check for exhaust leaks at the AIR ports, looks like the holes are plugged with pipe plugs.

when I first heard your video it sounded like an exhaust leak to me

1 thing for sure - your AIR exhaust ports are plugged with pipe plugs which are 27 threads per inch, the holes are tapped with 1/2"-20 NF which is 20 threads per inch. Get you some 1/2"-20NF x 3/4" socket head set screws in there and use a bit of high temp sealer.

Sounds to me like the timing chain is rubbing......the noise is not staccato like a lifter or rocker arm.....

Jebby

Did those engines use the Nylon POS sprockets?

I believe so...the hyd 390/400 was just a passenger car engine....but it stands to reason that somebody would have changed it out when they changed the cam......but it is a consistent sound.......
I would pull the cover and have a look.....

Jebby

They sure did. Here's mine after about 45 years of use. Kind of scary...

http://i21.photobucket.com/albums/b2...psb8078ae1.jpg

73 or so I worked the back of a Chevron station. I must have changed out 1 of those a month. They get real ugly when the teeth shatter and shred all over the pan. 454 pass side manifolds liked to crack as well.:lol:

I know what you mean. I probably could have lifted the chain right off without unbolting the gear...:lol:
My neighbor back in the 70's had a 71 454. He came home from work one day and decided to wash it including the motor. He sprayed the hot engine with the hose and promptly broke/ cracked both exhaust manifolds.:crazy:

IMO that's really a possible thing. I still think that cam has walked which could be something goofy on the cam sprocket, bolts or worn in the block.
I know what CanadaGrant is saying and the theory makes sense and yes they run a taper or what have you but in my eyes, this cam is more than what's was engineered into it. That lobe on the left actually looks like the lifter is cocked...really bad unless it's a shadow.

More tear down by the looks of it to find out.

they were all passenger hi pro blocks and shared many parts,
you make it sound like the 390-400 was a joke

Likely nothing with the noise....but in that pic with the one rocker off...are all the rocker nuts in the normal running position? Because if they are we've got a pushrod length/valve height/rocker geometry issue going on too. Those nuts are way out past the end of the stud.

JIM

They were plenty strong but nothing special....and you can take my words however you want....I owned one and it was great but no L-88....
I call it how I see it.

Jebby