Another old goat speaking from personal experience here - no expertise claimed by me. Seriously.
- Speaking stock for stock, I've always found the 383 wants a little less timing than its' 440 big brother -
in any mode. At idle, a bone stock 440 likes at least 10BTDC; the 383, a little less, but more than
5BTDC every time I had one in front of me. 7-8 seemed to work usually.
- The stocker 383 seemed to like an "all in" setting in the low 30's BTDC (with vacuum advance plugged
off, of course) while the 440 always seemed to like a little more, like 35-36BTDC.
(This is the way I typically set timing on these engines - at about 2400RPM, I go for the best direct vacuum
reading and the smoothest running, which determines the "all in" setting - and whatever that leaves the
idle timing at as a result is typically acceptable, too. The engine will tell you where it's happiest!).
(Either engine will happily motor on down the road with the vacuum advance disconnected, by the way.
You'll suffer some fuel economy and maybe even a little cooling from doing it, but it's fine otherwise).
Yes, many of the Chrysler factory distributors have adjustable vacuum advances on them (the electronic
ones especially) and a simple little hex wrench (I'm thinking 3/32" ??) is all that is needed.
Once you get your base timing figured out, then you can start experimenting with the vacuum advance -
but know that sometimes without some modifications, that rascal will never "settle down" to an acceptable
operating state - the engine will be "jittery" at cruise speeds and will "rattle" upon letting up on the throttle
to coast/slow down.
Yes, that's what you're hearing if you hear it at that point - it's pre-ignition where it's not welcome by the
engine. It most likely is caused by the vacuum advance but as others have said, it can also be caused by
an overly lean condition, too - and if the engine is running hot, well, that just makes the whole mess worse!
Today's crap gas, along with whatever modifications have been made to the engine, how old it is,
what it's sitting in, all sorts of factors lead to detonation and pre-ignition.
In my own '68 GTX with a "semi-warmed" 440, I run the Mopar electronic ignition sans vacuum advance
as a result. The car is happy (but a gas hog, admittedly) and there's no staccato music under the hood.