my experience is that some some are easier than others. some i've never been happy with. I think its a combination of a lot of things ie fuel, carby, ignition

i can also say that i've never had a vacuum advance that seems to work (made no difference to timing when it was blocked off or not)

6-8 seems to work for almost any holden motor. maybe your balancer has spun. i had an HZ 253 performance engine that suffered this. carby bloke i use reckons he's seen quite a few of it over the years