I don't think you know what you're doing, lol, or at least not how to explain it properly. First thing, Mikuni needle clip slots are counted from the top down. Stock position for the '78-'79 carb set was the middle or #3 slot. If you mean you lowered the clip to the #4 slot then you haven't lowered the needle, you raised it and made it richer. If you were counting the clip slots from the bottom up then yes, you did lower the needle one step from stock (the #3 slot) and made it leaner. But, with a richer Z-6 needle jet, things may still be too rich. Yamaha had to use the smaller Z-2 needle jet to get the 135 mains to work without causing break-up in the upper midrange, and it just barely does that. Any increase in main jet size, even one up, will cause that break-up unless the needle is leaned a step.