Suzuki and Kawasaki are the worst for that. TL1000 shows 11,200rpm but infact limiter cuts in at 10,000rpm. ZX636 will show 15,500 but cant even manage 13,500 when measured.

As for the NSR If you have checked the earths then I would swap the tach, they can be quite fraglie and can give up the ghost (race kit suzuki ones always die). Its very hard for one to read high due to a wiring fault as its triggered by the PGM, more often a tach will read low if something in the wiring is buggered, so you dont want to know what I would change after that.
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If I have to take the carbs off once more...