My 300 pro started running bad at the weekend. It sounded like 'pinking' or over heating. The bike stopped after about 30 seconds of running like this. I can notice a puff of air coming out at the bottom of the cylinder when I kick it over. Is this likely to be as simple as replacing the base gasket or am I looking at a more serious problem?

If you are lucky it will just be the gasket. But when it stopped it could have knicked the piston.

common problem on the 300 but Gas Gas UK can supply you with a better gasket.

also check the cylinder studs when you have it off,my front right one chewed the threads out.

magnesium cases are soft so used helicoil.

that and the new basegasket which is thicker solved the problem.

also when youre done retighten cylinder nuts after a couple heat cycles.