Well ... slow going but making progress!
I kept dropping idle jet sizes, all the way down to a #10 (the smallest available for a 34mm Mikuni VM!) but the bike kept chuffing clouds of fuel smoke out the pipe when idling. Less smoke with the #10 than with the larger idle jets (e.g. the #30, the #25, etc.) but still quite the fog machine. I triple-checked the float level (good), visually reinspected the float needle and seat (good), confirmed the floats floated (they did - no holes), and that the float needle actually shut the fuel off when necessary (it did).
??
I was ready to throw in the towel, but figured even though the bike with the throttle slide closed is 'technically' on the idle circuit only, leaning out the needle jet / jet-needle circuit might teach me something. So I swapped the P8 needle jet for a leaner O6 needle jet I had in my jet box. And ... voila!
After I installed the O6, I fired the bike up with the #10 idle jet. There wasn't a whiff of fuel smoke out the pipe. It idled for crap so I reinstalled a #22.5 idle jet (it was handy). I got a decent idle (with not a whisper of fuel smoke), and a test-ride showed that the #22.5 is still too lean, but that we're close!
I've had the carb off ~12 times throughout this whole process (I only make one change at a time) and am happy to report that I'm optimistic again!
I have a P2 and P4 needle jet on order (the P8 is clearly way too rich; the P2 and P4 are roughly halfway between the P8 and the O6). Next step will be a #27.5 idle jet with the O6. Maybe the O6 needle jet is the ticket, but if not I'll have the P2 and P4 on-hand.
I reckon the lesson is that while 'technically' the bike's only on the idle circuit with the throttle slide closed, if the needle jet is WAY too large the signal from the engine's intake can be strong enough to pull fuel through the needle jet (enough fuel to mess up tuning of the idle circuit). I suspect that this might especially be true when you've got a big (500cc) cylinder pulling through a relatively small (34mm) venturi.
I'll keep folks updated, but again, optimistic again!
