Erratic timing during vacuum advance

Pretty easy to tell if an engine is going lean just by putting your hand over the carb a little while throttling up and if it seems to clear up then you know.
If it goes flatter when you do it then you know it's probably going rich, or the problem is somewhere else.
Of course exhaust smell and plugs can verify that.
Not everything requires a special tool.:D

Also you want to make sure you are not over advancing at full advance with 18 initial as well as being sure the advance weight springs are not so light they let the weights jump around at whatever RPM's you are seeing it at.
Vertical slop in the distributor shaft or a loose timing chain can also cause that.

Remember the phrase "Trust but verify" in unknown engines.
All the above diags can be done with ZERO tools save for a plug wrench and timing light.