The only way to tell if it's valve float for sure is measure seat pressure. What was the air/fuel ratio, and fuel pressure doing. Most of the time when it's valve float the motor will run great then just won't "wrap up" anymore. It's like hitting a brick wall in the power band. My 468cid Big Block makes 540 HP at 6800 RPM and makes 210 at 7000rpm, that's valve float. When it was on the Dyno it sounded sick as hell when the valves started floating when it's in the truck and I float the valve it acts like a rev limiter but I can't hear the sound it made on the dyno.
