Originally Posted by leathermang

Those are classic symptoms of valves set wrong... ( too tight, thus elongate when warming up and do not seal at the valve seat..... so much of the cooling of the valve is from convection to the head... that it is like a microphone scream... a vicious cycle.. not pressed against the head... heats up faster, elongates , presses even less, heats up even faster... a feedback cycle... all going the wrong direction...
better check them fast because you can burn them in a heartbeat...