It is going on a 2001 x5 4.4i so it is exempt from my local emissions testing as Illinois EPA only test cars ten years and newer. I still believe it is going to pass as CNG is a cleaner burning fuel over gasoline.

I don't think it is a true carburetor as the CNG is already in a gaseous form. It is called a mixer and the system delivers the CNG using the same fuel injectors installed in the vehicle. The system only costs $599.99 plus you need a tank so for around $1000.00 I think it is a viable option over $5.50 permium gas.

Below is their FAQ


EWS Conversion Kit
These kits are low cost and easy to install.
This kit is available for all gasoline engines including all EFI throttle body or multiport, carburetor, and even turbo or super charged engines.
This is how it works:* A mixer (venturi) is installed in the air intake before the throttle body which provides single point fuel injection.* An oxygen sensor on the engine's exhaust is monitored by the CNG controller which controls an electronic valve (stepper motor) installed on the low pressure CNG fuel hose going to the mixer to provide the proper fuel ratio at all times.
On all gasoline engines the combustion air has always been controlled with a throttle plate so it makes sense to bring the fuel in with the air letting the oxygen sensor adjust the fuel ratio.* This provides the best mixing of the gas and air which provides excellent engine performance.* The engine idles and runs the same as it does on gasoline without the intense programming and adjusting needed with sequential injection conversion kits.* There is more information on sequential kits above.* The mixer does restrict the air flow some when running on gasoline, but not enough to effect the performance or fuel economy.
The instructions are written for the car owner to install with many tips, "how to" information and a trouble guide.
No programming is needed as we have each controller programmed for your engines requirements.* Computer cable and software is available if desired.
All you have to do is adjust the idle screw on the regulator and observe the indicator lights that let you know it is adjusted properly.
Our controller features self adjusting electronics which always provides peak performance.
The control on the dash indicates which fuel you are using and how much CNG is in your cylinder.
This kit allows you to run on either gasoline or CNG with the push of a button on the dash.
Our kits for 8 or 10 cylinder engines come with high output regulators for more power and better performance.* You can upgrade your smaller engine kit to a high output regulator for an additional fee.
If you run out of CNG you will begin to loose power so you just push the button on the dash and you will switch to gasoline and continue driving. *You should always refuel before you run out of either fuel.* Read the features of both our EWS and Millennium kits to decide which kit you would prefer.