Come on Larry, you've got to allow for a few word choice slip ups in a group
like this.

I'm sure he meant "un(completely)burned" fuel. It's the ash of partly
burned fuel when the engine is too cold to promote full combustion that
causes the problems. It doesn't even have to clog the injector enough to
stop the flow of fuel. A little clot of carbon on a nozzle can break the
spray pattern and turn it partially into a stream which has less surface
area for its volume and thus burns less completely leading to further carbon
build up.

--
Roger Long