Forms of carbon in incomplete combustion When hydrocarbons burn with little oxygen, carbon is produced. What form do these particulates of carbon come in? 

Is it a simple molecule like $\ce{S8}$, or a small fragment of a larger structure, like graphite? 

Why is this form more stable than others?
It heavily depends on reaction parameters (oxygen/oxidants availability, temperature, pressure, etc.)

In a free-air incomplete combustion at a (relatively) high temperature, you get mainly carbon-based molecules of various kind.

In a internal combustion engine, that usually works at high temperature and high pressure, you can also get nanoparticles of various kind, including graphite.

In a free-air, mixed, relatively cold combustion, the original small molecules, graphite and other carbon-bases compounds can further react with other molecules giving you larger molecules and/or can stick to larger particles forming nano or micro-particles of various kind. Predicting/modelling this kind of reactions can be very complicated.