Define terrible. Too rich? Wouldn't rev? After all a spot too lean will just make it rev forever (well not for ever but rev far too well).
I'm quite surprised on a stock bike the cans alone made much difference at all, I've swapped std cans for 3rd party so called race cans on strokers berfore and seen nothing noteably change apart from a spot more noise, and it is just a spot as well.

I presume the bike runs fine in standard trim?

It is possible if it's totally standard (and thus jetted very very safely) and you live somewhere warm it's very rich as it is and the cans have allowed slightly better scavenging thus richening it up that bit more. After all the std '28 cans are the biggest 2 stroke cans ever.
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