Originally posted by daveid
well i do have poor vacuum in my engine, and i have vacuum advance hooked up but im wondering if its even helping and deciding if i should really keep it hooked up or not. Just learning the whole thing on timing with mechanical and vacuum advance.

thanx
david

Some vacuum secondaries are factory set to start advance at 6" of vacuum and they're all done at 12" of vacuum.

Mid 60's SBC as an example, but other SBC's have a different vacuum advance curve.
Just depends on what you're trying to do.

A fairly radical cam in an SBC will idle approx 9-11" of vacuum so you have enough vacuum to work with as far as the vacuum advance goes.

Here's a fairly long article on timing.
http://www.jalopyjournal.com/forum/s...light=Got+Time

It's also on this board, but I can't find it.