Hydrogen burns with a clean flame because neither its end product, nor any intermediates is oily nor sooty. However, that flame gives off a great deal of heat for any amount of hydrogen that is consumed. Now, when the concentration of oxygen is too low, then any hydrocarbon whether alkane, alkene, alkyne, or indeed any other structure, will produce soot if it will burn at all. Conversely, in sufficiently high concentrations of oxygen, they all will burn cleanly; in fact if there is any soot present, that will burn too.
Now imagine a hydrocarbon molecule that contains as much hydrogen as possible, in a fairly generous concentration of oxygen. Compare it with a similar molecule with less hydrogen, or possibly the same amount, but far larger molecules; for example propene compared to hendecene. You will find that the smaller, or the more hydrogen rich molecules, or both, will tend to give less soot under comparable circumstances. You will need more oxygen to burn the larger, more carbon-rich molecules cleanly.
There are several reasons for this, partly to do with the behaviour and growth of carbon skeletons in a flame, but one reason is that when there is plenty of oxygen and plenty of hydrogen there is a good hot flame and plenty of gas and hydrogen- and oxygen-rich plasma that interferes with the process of building up soot structures from the carbon skeletons. That keeps them in a situation where they oxidise comparatively rapidly and completely before they can form any soot.
I hope that helps, but of course if it doesn't, please ask again.
