Isky had a problem with a engine way back, had a big power drop at a specific RPM and killed MPH, but didn't show up on the dyno.
The Spintron found it: a harmonic at a speed in between the increments Isky used. He tested every 500 RPM, viz. 5,000, 5,5000, 6,000, this was at 5,200 RPM and not tested under load.
The Spintron showed the valve springs hopping up and down, on & off the spring seats like a sparrow in a bird bath.
Better damper fixed it.
