While I'm not a mechanic, I have been around cars for over 40 years and have never heard of such a thing. Are you sure the rubber hasn't slipped on your harmonic balancer causing the difference? Dana 44 is good on the big block stuff, so maybe he'll see this and enlighten me. And you'll know what he's talking about.
Since you are talking about the timing jumping around and not the cam itself moving, I would investigate the distributor itself. There is a bushing in the block that holds the distributor shaft in alignment, and then there is the helical gear that drives the oil pump, the distributor plugs into it with a hex. The walking could be a worn intermediate oil pump shaft or a worn bushing, allowing it to twist up and down on its own. Next thing to look at is the springs within the distributor for the mechanical advance. If you pull the base plate the points/electronic ignition sit on, one of the two springs in there may have become weak, broken, come off its pin, so the timing itself would bounce around if not controlled by the springs.
The only way the cam itself will walk around is if a roller cam had been installed and the cam button wasn't installed, which keeps the cam aligned, something you didn't mention, thought I would point it out.
Thanks for that info, but its not a roller cam. No button is evident as i believe the stock timing chain cover isnt the one i got now, so its a possibility that THATS the reason?The thing about the timing change being noted was that before i stopped it from running itself apart, i tried several times to"time" it and noticed the big change, whether it was at a fast idle or running the revs up so i could see total timing. This mototr hasnt been worth the money and effort.
You can do a quick check on the tension for the advance by pulling the distributor cap and twisting the rotor. If you do it slow enough you will feel two different spring tensions, and it definitely should not flap back and forth other than about an 8th of an inch. Cams don't walk like this to alter timing, cam walk is front and back, which is why I mentioned the roller cam and button, lifters are set on the center and side of the lobes to keep them in place, so it would have to be a loose timing chain to make the timing jump around, but you alleviated that issue, so the only other timing item is the helical gear, which can move up and down, but not likely except how I talked about, so it has to be the distributor advance itself. Springs, which are about half an inch long, can break, so that's where to look for a problem.
I'm used to "walking" referring to front-back motion with the cam rather than timing/rotational issues, and is usually caused by endplay in the crank, which itself is sometimes caused by torque converter ballooning.
Someone used to make a sleeve with a set screw that fit over the dist. shaft. It would remove any clearance that would allow the pump shaft and gear to move up out of the bushing and change the timing.
There should also be a shim at the bottom of the distributor that can adjust end play, but I think it is more mechanical advance related right now. More than a degree or two jumping around isn't anything minor, and if it is as great as you said, keyway bouncing around that much would have to shear the woodruff key all the way off.
Is the deviation in timing more advancing than retarding? Have you driven it like this? Does the engine 'ping' as you lightly accelerate? Do you have a tack to observe? If so, is it 'stable' and not significantly higher? Do you have a functioning ammeter? I will await further input but suffice to say there is a method to my madness. I had similar experience about 10 months ago with B-engine and, yes, I did fix it permanently but to this day I have yet to figure out WHY it did what it did. :frusty: