I have a RV-12 with a Rotax. The engine has 2 carburetors that require balancing. The balancing is done with a tool that reads the manifold vacuum of each carburetor. With that information you can make mechanical adjustments to have both carburetors producing equal power.
A typical method to balance is using water columns, oil or some other fluid that is connected to the intake below each carburetor. There are electronic balancers in the $100 range with several LED's to tell you they are balanced and "OK".
I searched the internet and found a DIY project a fellow in the Netherlands designed to balance 4 carbs on his motorcycle. We communicated via email discussing using his design on the Rotax. I have built one at a cost of about $50.00. Not to difficult if you have a little electronics background.
The unit has 2 hoses to connect to each carburetor and a 5 volt USB cable for power. Internally it uses 2 automotive MAP sensors to read the vacuum. It will read in inches of mercury or millibars. The accuracy is way more than is necessary for my purposes, about 1/30th of an inch of mercury.
You can find the DIY plans here;
Mine being used reading in milibars;