[68gt500]

Depending on your camshaft you could need much more advance, I would try up to 16 degrees.

Keep advancing it until the idle smooths out.

Just make sure not to exceed 36 degrees at full advance.

Alternatively, if you have a vacuum advance hook it up to manifold vaccum, that will give you more advance at idle.

Spotting the faulty cylinder is pretty easy if you use an infrared thermometer to check exh. temperatures.

The rule of thumb is opening the idle screws pretty close to 1 turn - if you need more or less that´ll indicate too small or too big an idle jet.