Question edited for clarity, and add maker, toss in some CAPS.
Question moved to model category.
Mating surfaces not flat and true.
Reinstalled without a new gasket.
Under torqued or unevenly torqued.
No gasket goo used.
https://www.google.com/search?q=2001+Yamaha+YZ250F+valve+cover+gasket+leak
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