So... I played with a clock doubler circuit a little bit. And obvious the 74HCT86 is too slow to generate a stable 14 Mhz clock.
Here is the 14Mhz waveform with two different 74HCT86 IC:
First one:
Second one:
First one is almost good, but the 4th pulse is too short.
The second one is just too slow to generate the 4th pulse.
Time to buy a few 74F86...