[5valve]

About jet block gasket. Had an '08 300, with finicky carb...engine completely rebuilt.
Could never get it to idle right, too low, too high, intermittent, cutting off.

Tested jet block leak as per YT videos, seemed OK.
Bought chinese carb copy but never installed it. Sourced the jet block gasket from it, put in original carb...aaaand...nothing changed. So replacing this gasket ment zilch.

It coud be atomizer being oval, bowl leak or just tiredness of casting from years of use.
Installing copy carb was a better choice for you.

I did get chinese ktm replacement starter motor, needing only 180 degree housing twist to crank correctly, and so far starts like a champ, instantly, for 1/6th price.