When I got my bike I noticed it already had a HD muffler on it,until I latched on to this web sight I didn't know about carb mods,it has a great sound and that was all I cared about,I don't know if the last owner re jetted the carb or not,or if he played around with the white washer.
I supose I shall have to have a look inside the carb,how thick is the white spacer to start with? what is the standard needle size?, I do get considarable backfiring,even while cruising along at 60mph very small ones but they are there.   Today I screwed the mixture screw in a turn and took it out, I cranked it up to 85mph
it might have done a bit more but I ran out of road,no backfire on the up,but coming down was another story, do you think it's been modified?

If 85mph was flat out max then that's what a stocker with unmodfied carb and pipe will do even when hauling my 86 kg of lard around.  I would like a shade more top end but I'm not sure I'd feel safe. ("Safe? Safe? Of course he's not safe. But he's good"  Spoken of Aslan in CS Lewis's Lion Witch & Warderobe)