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I have always left mine unhooked because for some reason when I run a bigger cam than stock the motor will skip while at a steady RPM while traveling down the road. Unhook it.. plug the carb port no more skip.

Why does that happen?

With a fixed plate does that do away with the mechanical advance?

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.