My issue is that after driving for about 30 minutes at 50+ mph occassionally I'll start to develop detonation. Left alone it will continue to worsen.

The first time this happened I found a faulty air intake temp sensor causing the thermac to remain closed no matter what the engine temp was. Fixed that issue.

Today I had the same issue and in a mild frustration tantrum kicked the throttle and everything went back to normal. This "fix" was repeatable so I don't believe it is a fluke.

Is it possible for the vacuum advance to "stick" a little?
Or if the carb somehow creeps back onto high idle will this cause the same sympotms? And if so what would the remedy be?

 

Sounds almost like it is more of a carb problem than a timing problem. Almost sounds like it's loading up with fuel.

I had bent the vacuum advance on the HEI distributor that is now in my K5. I could set it to run great at idle, but it would fall on its face under load. I could also adjust it so that it would run good under load, but would stall at idle and was VERY difficult to start. Visually, the vacuum dashpot looked good. I used a hand vacuum pump to pull a vacuum on the distributor while I watched the timing with a light and it wouldn't move regardless of the vacuum applied. But that wasn't an intermittent problem.

The vacuum advance is cheap enough as well. While you are at it, check the condition of the pins and bushings for the mechanical advance.

 

With a vacuum I can pull 12" but it slowly bleeds down to 6"ish then holds steady. Had a local mechanic double check me and inspect springs and weights. Springs weights and pins look good to him.

I unplugged the wire from temp sensor to dual capacity pump and for about 1/2 a day thought i'd found the culprit. Probably just masked the real cause. Tuesday a replacement vacuum advance comes in. At that time will check distibutor for end play.

Plugs show a slight lean mixture. Heat range in proper area on strap. Light brown ring 1/3 around the ceramic. Verified by mechanic as well.

It was suggested that maybe a stretched/worn timing chain could cause this but debunked by another friend as it would be notcieable at idle as timing swings back and forth erratically. While it does idle up and down, the intervals are not regular but usually very long, 15+ minutes. If it was the chain wouldn't the idle swing more frequently?

I'm still leaning towards a vacuum leak of some kind but 2 cans of carb cleaner later I'll dammed if I can find it. Will blocking off vacuum to the charcoal canister isolate that as an issue? Is this system under vacuum during cruising?

If the ignition system seems to be fine and proper, Isolate all credible sources of vacuum leakage, could the dual capacity solenoid or adjustable part throttle be the culprit? Surely I don't need to rejet,rod...etc this reman carb?

 

Update: Upon inspection of distributor...Magnet shattered into three pieces, shaft bound up in housing, one spring not to spec. R&R'ed and so far so good. But of course 2 maybe 3 minutes after initial start up..Alternator bearing crapped the bed. All in all productive but expensive day.