After some measuring and looking at the very helpful JetsRUs
"Identify That Jet" page , it looks like I have a
Dynojet DJ type jet.
In order to measure the jet bore diameter, I inserted a tapered needle covered with Prussian Black (sharpie) into the jet and measured the impression it left with the micrometer. Got around 1.09mm / 0.0429 in, which according to
this page makes this a Keihin 112 or Dynojet 108, which is somewhat pleasing to me because it aligns with the hypothesis that the bike is running too rich at WOT, and stumbles when it hits the flat spot around 8000rpm, and what you guys have been saying about mistaken rejetting attempts by the PO. In this case especially suspect because they just had a slip-on exhaust. I still need to see whether the snorkel is there but airbox seems stock aside from that.
The
stage 2 kit from Dynojet comes with a max jet size of 100.
The stage 3 kit comes with a 110, so it's conceivable my measurement was on the narrow end, and this is what the PO got. Dynojet says this is for "mildly tuned machines using individual filters or velocity stacks".
Anyhow I'll go ahead and get a 96 and 98 (keihin 100 i.e. stock) to try out. Figuring I should stick with Dynojet if I'm not going to change the needle, but who knows if that matters.
Also verified the chokes are in good order (not much to go wrong there) and the needle jets are varnish free and correctly oriented.