Back in the day when people used to fit Yoshi big bore kits and cans etc, it was established by Pops Yoshimura that Honda purposely set the redline considerably lower than the motor could take. This was apparently to avoid too many engine failures. The first part of the standard motor to show it's limitations were the valve springs, rev them too high and valve bounce would occur. The bottom ends of the Honda fours appear fairly bullet proof. So along with Yoshi pistons and cam and valve springs the 750 bottom end could easily take 11,000rpm, and the 400 four a dizzy 14,000 rpm without any serious failures. I regularly used to thrash my 750F1 well into the red line with stock valve springs and never blew it. Provided the motor on your old K is in good condition, I see no reason not to ride it as Mr Honda designed it for, hard. But, if you aren't sure about it's condition, that's a different matter.
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