Sorry for commenting on a 3 year old thread, but I have worked a little with balance shafts on humongous machinery and wanted to post a little for posterior knowledge.
A single balance shaft on a single-cylinder doesn't really do much. It mostly just phase-shifts the vibrations so they go in another direction. Back and forth instead of up and down, like.
Dual balance shafts is another thing. They cooperate in the right direction, and counteract each other in between - and if you put them slightly offset each other vertically they can even mitigate some rotational moment.
But balance shafts also have another interesting effect - they can reduce the bearing load. There are actually two different optimums - one which gives the least vibrations but a bit higher bearing load, and another that is the opposite.
So when I see that small engines have a single balance shaft, I usually assume it is to allow it to manage high revs by means of lowering bearing forces rather than reducing unpleasant vibrations.
There was a bike back in the 90's(?) - some european single-cylinder 2-stroke 250 cc enduro (cross?) - that had a balance shaft and managed to output 70+ hp when all the balance-shaft-less big-brand competitors only gave 55 hp or so. Because it revved to like 12000 when the others quit at 9000 or whatever.
