Re: Ways to test the Vacuum Advance?
With engine idling, make sure the vacuum line to the dist has full manifold vacuum. If it does not, then move it to a port with full manifold vacuum at idle. Also make sure the rubber line itself is in good shape. If it exhibits any cracking or is loose to the fittings, then replace.
Now, with engine idling - simply watch the timing mark. Connect, then disconnect, then connect again - the vacuum line, while watching timing mark. The mark should jump substantially up and down as you connect/disconnect the vacuum line. As you connect the line, the mark would appear to go to top (or above) timing plate. When you disconnect the line, the timing mark should return to your base setting (normally around 8 degrees).
Here's a very easy test - remove dist cap, connect long vacuum line to dist canister and suck on the other end. You should be able to visually confirm the canister mechanism working back and forth.
I would also remove rotor and check centrifugal advance weights. They often are rusted up and will stick. So, take them off, don't lose the little springs, clean everything up real nice and lightly oil, then re-assemble and try setting your timing again.
Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
Bought my first big-block Chevy in 1970