Iv'e been doing this dirt bike thing for over 3 decades and in the weekend i had a new experience
I had a DNF (that in itself ain't unusual) but the cause was
I had a plastic breather hose on my MX 125 fuel tank , the hose ran over the crossbar pad and under the tank .
The problem arose when the plastic breather pipe found its way onto the GMC expansion chamber
the heat caused the plastic pipe to melt together causing a vaccum and stopping fuel flow.
In hindsight a shorter hose would have fixed the problem but on the upside at least it didn't act as a fuse