Setting the timing with a vacuum gauge isn't really a great method--where the timing is set to make the highest & steadiest reading on the gauge sitting there idling,probably wont be "right" for driving loads..you'll likely get detonation and pre-ignition pinging,and have to back off the timing..
You could set the timing "by ear" and probably get the same results...advance it till it either pings slightly under a good load and then back it off just enough to eliminate any pre-ignition or pinging,and then see how it cranks with the engine at normal operating temprature,it may need to be backed off slightly more to let it crank over without binding..
(I've had a few small blocks that ran sweet with the timing over-advanced--but to get them to start after they warmed up was hard--one truck I wired a toggle switch to the coil so I could get it cranking over fast with no spark,then flip the switch,to avoid hard cranking and kick back--many racers use this trick too)..
Vacuum is afftected by carb adjustment ,engine condition and timing,and when you change one setting you'll often have to re-adjust another...
If you have a lumpy cam,the vacuum gauge wont be much help,the needle will dance up & down, and not give accurate readings..
I've done some reading on this subject and almost every article says in the long run its best to use a timing light to set the timing,not a vacuum gauge--I tend to agree,but I've had a few engines that ran much better setting the timing "by ear",maybe the balancer had slipped on the rubber and made the timing marks innacurate,or my "mods" to the carb and distributor's mechanical advance weights made it run better with the timing advanced past stock settings--or perhaps the timing chain had some slack?....
Probably be best to get a good balancer with the right markings or a degree wheel (or tape) and set the balancer up so a timing light can be used..
