Forms of carbon in incomplete combustion
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.