Dang! 16K RPM...... that would be something to get out of these motors.
I thought the valves floated after 10.5K with the Action Fours cam and springs I had in my 76 811 back in the day. Maybe it was the points floating?
Action Fours had some fabulous parts, if their gaskets sucked. They were the go-to for the racing bikes we made back then. Their cams were notorious, though, for short life and 'bouncing' the intake valves from too-steep opening ramps, mostly because they thought that longer duration needed to also have wider valve openings. After the first 2 years they backed off the amount of lift (smart) but then their cams got soft for some reason (not so smart) and I haven't seen one in decades. If you had an early "Street/Strip" Action Fours cam, it opened the valve so suddenly that the 2 springs got into harmonic issues and bounced the rockers down the closing ramps of the cam above 9500-ish RPM. That is likely what you experienced: this disrupted the next intake cycle in the head by setting up pressure pulse moments back toward the carbs when the intake valves were jerking to different openings. The only thing I saw then that could fix this (it happened in roadracers, too) was those flat-wound spiral-like spring inserts that slipped in between the 2 valve springs, but they had to be run almost 500 miles before they smoothed out their mating surfaces, so they weren't popular on the tracks: heck, 500 miles was a race engine's lifetime on some of those tracks!
This was when Megacycle came out with their version of the [Daytona winning] Yoshimura "85 HP" cam that is now their 125-00. I've installed a bunch of those, all with great results. I don't think any of my builds make it up to 85 HP, though, because I don't normally use high-dome pistons like Yosh did, nor their fancy carbs. Or those beautiful megaphones....
By chance, did you also use the cast 811cc pistons? Those were most-excellent parts. Their forged ones, not so much: the thin rings could barely make a single racing season.
The 'newcomer' to those midget-car racers who got me entangled with them had a box-stock 750 engine he had made for his new car, and it had nothing more in it than a cam (he wouldn't say which one) and a 4-into-1 homemade (looked it) intake manifold with a Carter 2-bbl carb on it. He was running 12k RPM on the straights with just "...some shims under the valve springs..." and was heads-up with the little Offy-equipped cars on the shorter tracks. He would sit under a shade tarp and sip something cool between heats while the Offy guys were tuning, twisting, hunched over their hoods trying to beat each other. He never got better than 3rd place, but he sure had a good time at it.
