no way in heck that this timing/ carb relationship can happen
i don't trust "dial up" timing lights and there have been problems with various lights with msd. If you can, bypass the msd until you get things settled down
the idle screws not having effect could be a few things
too much fuel pressure driving fuel past the needles, too high float level, or other problems causing the float level to be high
the metering rod springs may not be seating at idle, remove the top covers and see if this is so.
Has the carb been hacked? Ie someone changing jets/ rods/ springs?
