I'm afraid I already know the answer but asking here. My 69 wildcat has been pinging and stumbling at WOT off the line for about 25k miles now. Carburetor rebuild and adjustments don't help. Power timing procedures don't help. Doesn't use any oil and no other engine troubles. Meticulously maintained. I checked the slop in the chain by turning the crank back and forth observing slightest rotor movement. Seems like about 8-9* of crank movement before rotor begins to flinch. I should probably compare to my fresh 350 chain slop. Would this slop in the original nylon gears and chain after 78k be causing these engine performance troubles?
I would check the distributor the advance stop bushing as there made of rubber and deteriorate. If it is missing will let the mechanical timing go way over 28-30 total, causing severe pinging eventually destroying the pistons. I personally seen it happen to many 400-455 engines through the years. A $1.00.part ruining the engine.
You know the KISS principle? You have to keep trying the simple stuff before you get invasive. A sloppy timing chain should not cause the engine to run that poorly unless it is really bad. With a light on it, does the timing mark flutter around or is it relatively steady? If it is relatively steady, I'd look elsewhere. What's the ignition timing? Initial, total, and are you using vacuum advance? What's the engine vacuum fully warm in Park? 1969 engines had pistons that were in the hole quite a bit. That will make the engine more susceptible to detonation/ping. If the engine runs on the hot side, and is lean, that will make things worse.
Checked timing just now. 3* initial and 28* total at 2500. vacuum advance is inoperable. It holds vacuum pretty well but doesn't move when i add vacuum with my mityvac. I will leave it disconnected for now. Timing advances smoothly with throttle. If I advance distributor more it pings a lot on all premiums I've used. It would just be nice if it didn't stumble off the line.
First, the 69 distributor has 30-34* of mechanical advance. It's probably advancing beyond 28* at higher RPM. Set it to 0*, replace the vacuum advance, and hook it to manifold vacuum. That will advance the timing 14-18* at idle. That may eliminate your stumble. Put a vacuum gauge on the engine to make sure it does not have a vacuum leak somewhere. It should pull at least 18-20" in Park, fully warm.
My vacuum advance is inoperable. Doesn't budge with mityvac. Must be stuck. 18 inches of vacuum at idle off large port of carb in front.
I think of fuel first in this scenario. If you haven't tried it, run some racing gas in there and see if it still pings. I see you have identified an issue with your vacuum advance. Fix that first. Perhaps the diaphragm is leaking. Or as you said the mechanical bit could be stuck.
Diaphragm is not leaking but is now disconnected because it doesn't advance at all. I confirmed total advance is 28 or 29 tops with 3* initial. All in at 2500. Maybe that's to early and should have stiffer springs. So I guess I will do some work on the distributor this winter.
https://www.v8buick.com/index.php?t...stributor-1111260-6lb17-and-manifolds.160441/ 1967 430. Who knows, might have been modified in it's 56 year life.
Mityvac holds steady with vacuum applied. That seems to be not leaking or possibly plugged! Distributor is out now. I peaked at the timing gear and chain. I'm guessing it's original?
You should see the plate move when vacuum is applied. Either the plate is stuck or the unit itself is stuck.
Remove the VA, apply vacuum and see if the link moves. If it does, the advance plate may be binding. You need vacuum advance.