Its called a power jet modification, the info below is for motorcycles and is somewhat inaccurate as they can also be installed on Mikuni watercraft carbs obviously.
Mikuni Power Jet Kit: Turn Your VM Roundslide Into A Modern Carb
Price: $24.00
Make your round-slide Mikuni VM carb work like a modern carb: install the Mikuni Power Jet Kit.
Improves midrange power while adding additional fuel during WFO (wide open) throttle situations.
There's less chance of seizing and overall you get better, crisper throttle response.
What you're getting:
* The Mikuni Power Jet Kit comes with full instructions and all parts necessary for the modification
* Includes extra jets for tuning
* Prevents lean-running during extended full throttle periods, while boosting midrange power
* For two-stroke motors only
* Works with any Mikuni VM carb from 30mm through 44mm
* Very complete instructions
* Requires one simple machining step, although it can be done with a file and a little patience
If you want some extra oomph from your vintage bike or can't seem to get the jetting just right, the Mikuni Power Jet is the easy fix.
Note: ONLY works with Mikuni VM carbs on two-stroke motors.