Kawasaki Triples Club

Well that was interesting. Had a spot of bother with my H1 after fitting a set of 3 into 1 Pipers. It ran great for the first 40 miles or so then started misfiring and running onto two cylinders. I tried new B9's, then Iridium's to no avail but it kept doing it. Then I found some old coked up B8HS plugs I'd almost binned, heated them up with a blowtorch and burned all the crud out of them and tried those.
On first kick it sounded crisper than before and Hoowee deputy dawg, that bike flew down the road like it's never done before! Now it revs to 8K and has even more low down grunt.
Guess B8's work better with Pipers then.

Sounds like your mixture went rich. The 3 into 1's never really let the motor get completely into "sync" so you do get a lot of unburned fuel.