here`s a few things to check over. First off, do a spark test, if the spark is yellow, it`s weak, secondly, do a plug chop, run it at wide open throttle until it starts to choke, take your foot off the pedal and kill the ignition (but don`t lock the steering wheel) coast to the side of the road and pull a few plugs, if there white it means it`s dead lean, could be caused by low float level, lack of fuel pressure or bad fuel pump, possibly a clogged fuel filter.
If the plugs are wet and black, then it`s getting too much fuel, make sure the choke isn`t closing and check the fuel pressure.
However, if it ran okay before you advanced the timing, then I would go back 2 degree`s at a time until the problem stopped and leave it set there. vacuum drops to zero at WOT, so the vacuum advance isn`t in operation.