Cluelessintexas

I happened to nishiki 12 speed, not even sure if that qualifies as vintage. Shimano shifters, Shimano brakes, Sika surface tires. the serial number is KK 34853. don't know if this means anything. for that matter anyting remotely close to value. someone, please let me know.

repechage

Nishiki 12 speed bikes started arriving in the later 70's- Could be so many things, my guess not one of the higher value offerings to begin with.
As condition guides so much, as does size to an extent, not much to go on.

T-Mar

Nishiki had two sources that used serial number formats starting with a K. The more common source was Kawamura but their prefixes only ran from KA though KJ and they also used KS. The other manufacturer was Katakura who typically built only the high end models. If it is fact a Katakura manufactured bicycle, the serial number indicates a 1974 model built in November 1973. However, a high end 1974 Nishiki would not use Shimnao, nor would it be 12 speed. Still, given the age, it's possible that it has been upgraded. If you want a more reliable identification and assessment, you'll have to provide photographs.

wrk101

For every really good Nishiki, they made hundreds of average bikes. Not knowing more, I'll guess entry level. Depending on condition, it could be $25, or $125. Tires have no bearing on value. Condition is everything, along with the quality of the frame material and components.