I added an adjustable Vacuum advance to the billet MSD distributor when installing the Mummert heads. I don't remember part number. . The can was Morroso.

I don’t have a part number for an adjustable vacuum canister for the MSD distributors.  Depending upon the amount of vacuum, the supplied MSD vacuum canister has the potential for as much as 14° additional advance over whatever amount of mechanical advance you have the engine set up for.  There are some Accel vacuum advance canisters that are adjustable but I’m not aware if those will interchange with the MSD vacuum cans or not.  A call to MSD might answer your question on this. 

Lorena, Texas (South of Waco)

Hey Ted would you by chance have a part # for an adjustable vac advance can that will fit my MSD 8383?

Because the idle speed when using any kind of aftermarket camshaft can vary, the initial timing is not going to be a specific value.  I'll recommended setting the timing with the engine at ~3000 rpms or where all the mechanical advance is in with the vacuum advance disconnected.  Setting the total mechanical ignition timing so it’s at 38° is a good starting point.  This may require you putting a 38° mark on your damper.   Once you set the total timing, the initial timing at idle is expected to fall in the 12-15° range with the vacuum disconnected.  If the carburetor is right and the vacuum is hooked up to a ported vacuum source and not a direct vacuum source, the ignition  timing at idle should be the same regardless if the vacuum is hooked up or not.

Lorena, Texas (South of Waco)

Awwsome thanks Ted I will try those springs. What do you run your initial timing at on a mild cam and headers? Also I just put on a 600cfm summit carb and I really think my engine likes it better than the 500 avs2 eddy. 

For most Y builds and using the MSD #8383 ready to run distributor, using the lite silver and blue springs in conjunction with the blue bushing proves to be the ideal combination for both performance and economy.  I use ported vacuum on most Y builds which eliminates issues with inconsistent ignition timing at idle speeds.

Lorena, Texas (South of Waco)

I tried several different combinations with the springs and advance bushings. To be honest I never really noticed much difference in performance. My engine has a decent sized cam in it (236@50) and I did notice it likes full manifold vacuum on the advance as opposed to ported. I'm now running a custom built distributor and the builder recommended full manifold vacuum also. My suggestion is to try both and see what works the best.

Durham Missouri

Plumb your vacuum gauge into the cabin so you can see it when driving. Backing up here a bit, might first try disconnecting and plugging the vacuum advance and taking a test drive or two. This is the way the mechanical or centrifugal advance is tuned anyway, it's always adjusted first.

Full time manifold to the vacuum advance is sometimes very useful for performance applications (big cams) driven on the street, as they often won't idle very well, and need a higher RPM etc. With lots of ignition advance the throttle plates might be able to be dialed back.

But if you like it there regardless of your application, it won't hurt anything. It's a very misunderstood subject. There isn't any load on the engine at idle. So 30° or more at idle wouldn't be unusual with it connected there. Keep in mind the ignition timing will revert to whatever the base initial timing is, as soon as you accelerate.

Vacuum advance is a load dependent mechanism. Mechanical advance is RPM based.

So a manifold connection isn't really going to affect the ignition advance when cruising on flat ground or, at wide open throttle or anything (much) in between etc. Just at idle. Engine will run a lot cooler at idle though, and in stop and go driving, this can be important depending where you live. It will not pass smog however, if that is a factor.

It can sometimes be difficult to get a steady idle RPM with a manifold connection to the vacuum advance. If engine idle manifold vacuum is low, or less than the # for full advance of the canister it may "flutter" and this interferes with the timing, which changes the idle RPM, etc.

In fact, if you read the old manuals from the 1940s, this is why the "ported" distributor vacuum advance connection was invented - that is, above the throttle plates - so as to ensure a steady idle.

Sometimes you'll hear it is an emissions or smog feature - it is true that retarding the ignition timing at idle reduces NOX, but it predates smog considerations by decades. I'm not sure when vacuum advance was invented, but the spark port was probably invented at the same time. Some makes used a manifold connection, some did not.

I think He means using the ported Vacuum hookup on the Carb rather than full time Manifold Vacuum. The Position of the Throttle Plates regulates the Vacuum available to Distributor Advance... 

  

RB thank you for the reply. When you say "Your carb should regulate when the vacuum advance kicks in at cruise, not under power or at idle."  
Would you mind elaborating on that a bit? After a couple of years of learning i am finally wrapping my head around total advance. Its the vacuum advance that i am still curious about.