My experience has been very long-term on my original '83 Voyager, and I still can't believe how trouble-free that bike was in it's 212,000 miles that I had it. On my current '85, with 45,000 miles, it still runs like a top, with no issues other than a slipping clutch due to the wrong oil by the PO. I never did take much apart on the '83 (if it ain't broke, don't "fix" it!). So, I have no idea how sloppy it's shafts might have been, but I didn't care because it ran just as well as when it was new. On my current '85 I bought two complete fuel systems to rebuild and shelve as spares, and also took the original off the bike for rebuild. It, too, ran like new, so there were no "crippling" air leaks on those shafts either. They felt pretty tight so I didn't remove them. As I mentioned elsewhere I used Seafoam as my cleaner so no seals would be damaged.
I have no suggestions why injection body shafts don't seem to have these same problems EXCEPT that all functions are being monitored by the DFI and being compensated for to some degree or another.
A rule of thumb is that the smaller the carb the more effect a leak has. If an 1150 cfm Holley carb on a 600c.i. Chev engine has just .005" clearance around it's shafts, there is absoutely no issue. But, shafts need pretty much the same clearance no matter what size the carb, so that same .005" clearance on these little Mikunis is quite significant.
As far as the leakage around the shafts at idle, it is really a lot more complicated than we could hope to convey in mere words in a few posts! Bascally, at idle, where most of the issues seem to be, the vacuum pulls the shaft hard against the body, removing most of that clearance that causes the leak, but not all of it. There's more to the pathway than just that, though. I could do a better job of explaining if I was in front of everybody and waving my hands all around and making ugly drawings on a chalk board!
Motorcycles don't usually get as many miles on them as cars before they are junked, so their carbs are not really intended to be rebuilt. Hence, no shaft bore bushing kits. That's too bad, because motorcycles tend to be abused by the Nervous-Neds out there who just have to keep blipping the throttle faster than a hummingbird's wings flap, and that really wears those shafts and bores out fast! Maybe if Kawboy can come up with the perfect seal replacements, I might find an old KZ carb on eBay to look at for the possibility of boring out to insert shaft bushings. I have full machining capabilities for small work like this and it would be easy to make up lots of bronze bushings and to bore/ream out the worn bores to be bushed. It's not simple, as the bores have to be relocated to original specs or the butterflies won't fit in their bores anymore.
This also brings up another issue. As the shaft bore wears, it allows the butterflies to move out of their position to give a good seal at idle, aggravating the air leakage issue! This probably accounts for more air leakage than the shaft/body clearance.
