Your vaccum advance is coming in and out,depending on your total advance curve and where it comes in,then in relation to engine vaccum.You can do alot with tuning a vaccum advance-even on a radical street car,I always run them to full vaccum port on the manifold and usually only add about 10-13 degrees to help idle and drivability and even a little better gas mileage.Its in at idle and at very low load situations but out the minute I gave it some more throttle.The vaccum advance operates by the plate being advanced by vaccum,so a fixed plate just eliminates the movement and the vaccum advance,it also gives you a rock steady platform for timing.I have to say I dont even mess with vaccum advance on my cars anymore lol,too lazy and theres no improvement in e.t. and I dont mind my car being a little nastier at low speed anyway.