base timing keeps advancing by itself...

if you have a old starter (just in case you break it) that you can use. do the kick back test.
set initial timing to 5* BTDC
start motor.
stop
advance by 2 dgrees
start
stop
advance by 2 *
repeat
do this till it labours or kicks back when starting
remove 1 degree
see if it will start, any sign of labouring
remove 1 degree. try again
no labouring
record your new initial timing
battery needs to be well charged....

regardless of if the balancer has slipped this is what it wants as its initial timing.
work from that when you recurve your dizzy and work out if the TDC mark on balancer is correct.

some risk in doing this but you do get an indicator of what it likes best.
only method that really truley works if you put on something like Webers....but works none the less for everything else.
yes it sounds like a "bodge".... but it works, provided carb is in a decent state of tune.

re set idle mixture once done for max vacuum or highest idle speed and then adjust back to 800-1000 or what is appropriate for your car on the stop screw

Dave