I know that the fire is still burning when the port opens on a two
stroke, so I am sure some of the fuel burns at less than optimal
conditions. I beleive that some fuel is blown through the cylinder
when the charge enters, adding raw *** and nitro to the exhaust.
While I am not sure if similar ills occur on four strokes, I suspect
that they are reduced due to the more "controlled" nature of the
four stroke timing. The exhaust is much hotter on a four stroke,
which could make any castor in the fuel cook more.