Yes, that is unburned fuel. Hydrogen burns off first, leaving the carbon behind. This happens when the air/fuel ratio reaches the "smoke point".

This is the kind of stupid behavior that gets laws passed. Then we all suffer.

Colorado has a law making it illegal to operate a vehicle that has visible smoke. They have a toll-free number one can call with the license number. I got a letter one day saying I had been reported. It was true, burning a quart every 100 miles will show.

R.