J-8 J-12 the truth be told

Evening all, hope you’re all well. Any of you used/using a Jupiter 8 or a Jupiter 12? If so, what’s the best f-stop on your lens as to overall sharpness. I have two J-8’s and two J-12’s and my black J-8 (number 0239925) is the best of the two and gives excellent results all the way, f/8 being the best. My 72 J-8 seems soft and needs f/8- f/11 to produce anything acceptable. My 88 J-12 is a cracker all the way, though f/8 f/11 seem the best. My 77 J-12 is a paper weight! Soft wide open and not really making the grade even at f/11-16. Are the variations above the norm (Russian quality control noted).
The film to flange distance has been checked and adjusted where needed (3 out of my 4 Zorki's needed adjustment),
Like to know your experiences/thoughts with the above lenses.

All the best.
B.

In general Jupiter (and other Ukraine made lenses) suffered from loose quality control. Hence, while good designs overall, it is not at all uncommon to have to go thru several to get a good one.

I agree that inconsistent quality control (or incorrect servicing of the lens) is likely to blame for the results.

If that lens is serviced and correctly collimated, it should be just as good as the German (Carl Zeiss) lens from which it was cloned.