My turn... I doubt it's the valve spring pressure. I run 105 seat pressure and around 350lbs open on a Lunati .600 solid lifter cam. I've ran those springs for 35 years and will continue to unless something changes drastically.

Did you check/cut the valve guides so the retainer doesn't bottom out at max lift?
Block decked any? Check pushrod scrub pattern?

If all that is good, I would agree with some of the others...ignition and or timing.



This might be pushing it a little.. but did you mash the pedal down all the way and check the carb?