This won't be easy, due to four carburetors, but it's simple.
You have two routes-slide cutaway or needle-jet height.
Notice carefully the decelleration characteristic. Is it lean-bogs- on acceleration, but doggy-rich-off throttle, I would cut a half-mm. off the top of the needle jet. Yeah, there's four and they have to be the same.
If the decel is crisp, I'd cut a half-mm. off the bottom of the slides, lowers cutaway in a roundabout manner. Replacement parts are available, but you're accustomed to machine work, from what I've seen.