Just my experience, but it's not from a KZ or the bike.
I once had 86 Toyota Corolla with 4-AG engine (yeah, twin cam one). Modified the engine to have 2 twin 40mm Mikuni's on it. Long story short, I can't remember what we were doing at the time, I had the exhaust off, just the headers on it. I think we were just making sure it start. Yeah yeah, I know. But hey, I was young and stupid. Anyway, flame shot out at the end of headers. Never saw that before. Found out my main was waaaaaaay too big, and as soon as you closed the butterfly, that unburnt fuel came in contact with air outside, just enough fuel to ignite, and created the flame (according to my mechanic friend). Exhaust was still hot enough to ignite it. According to him, when you have the butterfly open, probably was too much fuel to ignite it. Still remember this because I thought it was cool, but I knew it was not right and engine was running rough.
I also saw flame that came out of carb from V-8 engine ( I drove and modified various V8 car in the past) , but that's usually is a timing issue.
Not too sure any of the experience helps, but thought I'll post it here.
82' KZ750-R1 cafe racer style. Clip-on, Rear set, Fork-Brace, Mikuni Flat-slide (forgot the size), Kerker.
84' GPz750 (basket case). everything are in pieces.
89' ZX750-H1 (ZX-7 Ninja). Resurrection project are on going with my KZ750. Everything is stock.