My latest mod was to drill the vacuum piston (a/k/a slide) air hole transfer ports with a #39 Wire Gauge drill bit; a #39 WG bit is approximately 2.53mm. The stock air holes are supposed to be 2.5mm.
I drilled it by hand using a dremel chuck, as I didn't have a pin-vice large enough to accomodate the #39WG drill bit.
I'm not sure yet if this mod really accomplished much, although the throttle response is crisp. I don't like to do serious performance evaluations in the dark, although I did take the bike for a ride & it seems to perform well.
I don't see a need to go any larger on the slide air ports, especially with the FZ1 slide springs. I previously ran another set of slides that were drilled out with a 7/64" bit (approx. 2.78mm), & at the time thought that was too much for anything other than stock springs, or modified stock springs. I still do.
Under NO circumstances would I suggest drilling the air holes any larger than 7/64"; really, #39WG is large enough for the FZ1 springs. I would consider 7/64" as the upper limit, under any circumstance. It really doesn't take much to clean up & optimize the air hole. IMO Jenks is an idiot if he thinks 3.0mm air holes are called for.
Anyway, I'm gonna test this out a bit more when I get some time, daylight, & decent weather.
