Montesa were very cunning and some of the very early ones have several restrictors, first check the header for a ring and there's also an insert in the small part of the mid section, it can be seen when you take of the silencer.
Once all those are removed, purchase a Keihin pwk28, same carb as on the ktm85sx fit a JGH needle, 45 pilot and 120 main jet, drop that carb on and the bike will pull like a train in the mid range, a completely different bike.
after that you'll need flywheel weights and advancing the ignition, but if your going to go that far buy a 4rt
still looks a tidy little 315 you've got there