Be interesting to know track length of whos speedo is off or not, I wonder if the just put a flash with the wrong gearing in some, mines a 129.
The TY bunch is reacting in the same "Im never gonna get that flash done" way, mine is fine other than the speedo, big deal, it will be fixed soon. It starts fine, not sure why the OP,s backfire is happening, never heard of any others doing that, starts, idles fine etc. and no sign of kickback.
I had a good starting procedure before and wasn't too paranoid about kickback, but you could tell it wanted to unless you held the key, so its good to have that gone.