Well, that's a step backwards...

You have to match the piston to the bore for the proper clearance. Which means precision measurement of both.

I see he is riding the 300 SEF Factory in the endurocross series starting next weekend. It will be interesting to see how he does against the 350's and 450's in that class.

He didn't finish first either night. I think they need to put a big bore on the 300. Who knows, maybe they already have.

This is what I invented to test.  A tiny fuel filter. I will see if it flows enough air to vent properly.

It didn't solve the problem. The fuel boils just like the stock check valve setup did. It runs fine but somehow fuel still gets through the felt filter I added and drips on the front fender. New idea: I took the front fender off and put air filter foam up into the steering stem hole. Maybe that will catch the fuel before it drips onto the fender.

Took out the check valve...problem solved.  Rode in 90 degree heat the other day in slow technical wooded and  sandy conditions and if ever the bike was hot it was then.  No seepage down my front fender.

I removed the check valve on my 2020 300 SEF Factory and it solved the fuel boiling noise but it made the fender staining problem worse!

Also if you can't find the suspension info in the manual for the KYB suspension, you will find it in the French section. Clicker settings for comfort, standard and sport. The spring rate for your weight info is in there also.

That is a weep hole for the water pump.

 The weep hole lies between two seals - one that keeps motor oil from the water pump (the oil side water pump seal) and the other to keep coolant in the pump. When the coolant side seal fails fluid drips from the weep hole and eventually leaks out the cover.

https://www.motosport.com/blog/bad-water-pump-symptoms-on-a-dirt-bike-or-atv