Is there a way to limit the advance from the can? I know you can adjust the "on off point" with an allen wrench, but is there a way to limit the amount of advance?
yes, epoxy strips of feeler gauge on either side of the "notch" in the arm to limit its travel into the can but just me I think you have way too much timing being on manifold. keep us updated
Another thing you can do is to tighten the advance can spring down enough to where the beginning point is way late so that the vacuum advance is not "full on" at your cruise vacuum. Using one of those hand vacuum pumps is the easiest way to establish how much advance your combo likes. But trial and error with the allen wrench will get you there too. It will just take a bit longer to figure out.
And your mechanical advance does sound a bit much. How did you establish that as the best for your combo?
As a starting point, I use 32* mechanical advance and about 42* total with vacuum and tune from there. On the vacuum advance, I start a little low and add a couple of degrees at a time until the car starts to "surge" at cruise and then back it off until the "surge" is out of it.
How I got here -- I am at 18 initial (not centrifugal) based on vacuum reading and adjusting the dizzy. Once that was set, I knew I wanted to be around 38 total (initial+centrifugal), so I pulled the dizzy apart and adjusted the centrifugal advance down to 20 degrees (10 degrees in the dizzy). While I was in there, I removed the heavy spring, leaving the light spring to help bring the advance in a little quicker. This is a stock build 383 with 906 heads, so not the most combustion-efficient setup, so I think it can take a little more timing. Vacuum reading with the can hooked up is about 20". I got this point after the initial timing was set, hooked the can up, and tweaked the idle mixture screws.
I have a mighty vac pump, and will play with the can next.
What I think might be happening, my cruise is 2-2.5k at 45-50 MPH (not exact) but close. So I am getting pretty close to full advance at this RPM, with the light spring only it was all in by 2500, so 38 degrees. Add another 18 on top of that with the can hooked to manifold and that is 56 degrees of timing at cruise. Might be a tad too much. I think low 50s is what to shoot for baseline and tweak per your surging adjustment comment.
A few options -- bring the initial timing down some, and readjust centrifugal to get back to 38 total. Attack can to port vacuum so that it does nothing at idle (which I want more timing at idle to help with cooling tooling around in traffic), or adjust some of the overall timing out of the can with the shims.
Either way, removing the can and/or trying port timing should tell me something. Either the car behaves the same way (which would mean it is likely carb related) or something changes (which should tell me it is timing related).
The other part of this is I am getting a whistling sound from time to time at idle/950 RPM. Vacuum leak? Not sure, vacuum signal is very steady at idle. Tracking that down too.
All part of the fun.
