Distributor Vacuum Advance Can.........

....... cap it or use it?

If it’s a street car I would use it . But a street car means a streetcar not a race engine driven on the street.

E here berg has written a ton about this

Understand how the can works, adjust it and use it. Street car with HEMI, B or RB and big cam? Get one of these: VC-1 BB Big Block Mopar Low vacuum signal canister for the big cam, pump gas Pro-Street motors at: https://4secondsflat.com/Mopar%20di...low%20vacuum%20performance%20engines.htm

Check this thread out: https://www.forbbodiesonly.com/moparforum/threads/centrifugal-advance-vs-vacuum-advance.172034/

i use them and won't go back to full centrifugal on the street anymore. but, there is somewhat of a learning curve to set a vacuum advance up on a cammed up engine. i've used all kinds and in the end i haven't found anything more versatile than the factory 9 degree that came in the electronic conversion kits.

This and their mechanical advance limiting plate if you're going to play with initial timing so you don't get to much total mechanical advance.

with any factory distributor used on a modified engine, and even a stocker, one needs to get inside and play with the weights and springs.
every combination of the same components is slightly different, so using specs someone suggests will work, is just the STARTING point.
also be forewarned, this is NOT a 5 minute job..........
it takes some trial and error to get the right weight spring and weight calibration for your combination, THEN, you need to work on the vacuum advance curve.
just my personal experience with distributors over the many years. your mileage will vary.

What application, fuel used and motor type?

Street, Premium (93). Stock 440 rebuild. Thanks Cab!

I would set the total timing (rev up beyond the mechanical advance) to 34 BTDC with the vacuum disconnect and see how you like that and then hook it back up and try that also
What brand spark plug and heat range are you using? How much mechanical compression ratio and which heads, stock or aluminum?

Champion RJ14YC. The heads are stock cast iron. I do not know the compression ratio.

you need to change those plugs , they are way too hot

Try RJ12YC or something similar in heat range from NGK or Autolite

AC= GM
Autolite = Ford
NGK = Imports
Champion= Mopar
Verdad!

NGK = No Good Kind I used to work at a motorcycle dealership back in the 80's ... when putting in fresh plugs in a Suzuki, all the mechanics would ask the parts guy for a new set of of No Good Kind. We all got a chuckle. It stuck.