1979 Mikuni B38 has different pistons

Looking for advice from the experts! I have 1980 KZ750G, but with 1979 carbs. I've always had trouble with these carbs (replaced what was on it with Ebay specials) After tearing them down I see the vacuum pistons are slightly different, & the needles are not the same length and the holes on the pistons are different sizes. Stupid question..does this matter? I've been looking for a complete 1980 carb, or matching pistons but they seem to be hard to find. Thoughts? Thanks so much!

Yes, those things matter. The size of the hole affects how much vacuum it takes to pull up the slide and how quickly it responds. The needle controls the fuel mixture throughout the mid range, from about 1/8 - 1/4 throttle to near wide open.
It is hard to tell in the picture, but it looks like the slide cutaways may be different too. They affect low speed running.

Thanks so much, now I know how to proceed!

Yeah, there is no way those slides should be used on the same engine.

the BS38 carbs that came on the 1980 model had a few unique differences that differed from the 76-79 years, different pilots, needle, and jets - including the needle jet. there are other smaller differences as well.

you can try replacing the slides, but it may be easier to find a whole different rack in the end.