I find info and part#s on vacuum delay valves which delay the release of vacuum... such as for timing advance or other, keeping vacuum to the component for a short time after the source is expired.

I want one which works opposite... a valve to delay the application of vacuum, but release immediately.

Is there a such thing? If so, please help. This is upon request for a Mercury. What it really needs is to delay the advance on the dizzy, so advice on that would be helpful too. I'm not too keen on weights/springs changes cause/effect.
edit: stiffer springs?

Adjustable vacuum advance would be the obvious choice.

http://www.cranecams.com/pdf/272e.pdf

Thanks Vikki. That's actually two votes for the exact same thing. One is from the engine builder once the problem came up after the cam change (knock when getting on the throttle at cruise).

So what you are experiencing is knock when vacuum signal DECREASES with throttle opening...generally meaning either to lean a mixture or too much timing. You already have full vacuum signal, and full vacuum advance, at cruise RPM. Delaying onset of vacuum won't help.

Have you tried driving it with the vacuum advance plugged, or changing source from manifold to ported or vice-versa?

It's currently running with no vacuum advance. Seems OK like that, or at least no knock, but we'd like some more advance back. I don't drive it, and my bro doesn't drive it as much as he should IMO. It's a nice car. 1968 Cougar.
I think he tried it with manifold vacuum... i don't know. It was running with ported and a racing cam, but I changed the cam for street driving. It wouldn't go slow without riding the clutch before. Since the cam change it's much better.

It smells rich still, but he's running 66jets in a holley 650(?). It's a weirdly modified 379 from a 351W... chevy pistons, Mopar rods...
http://www.bacomatic.org/~dw/dehaven/dehaven.htm

it still goes like stink though. I'd just like to get the advance when it likes it and not when it doesn't. Do you think the adjustable advance kit will help?

edit: re-reading. you think the vacuum kit will not help? darn.
It seems like the ported vacuum actually increases at load. weird/imagined?

True. Which is why vacuum source matters, and why Pontiac used a TVS switch to change between sources based on engine temp.

Don't know much about Holley calibration, but 66 would be very lean primary jets in a Rochester with stock rods.

Get rid of the acceleration knock before adding vacuum advance. Lock out the secondaries and see if it goes away or gets worse. It should be slower, but if primary mix is good it shouldn't knock. If it's worse or same, check the accelerator pump. If it's better, be sure there is fuel in the secondary circuit.