Dirty Carb

I pulled the carb on my 06YZ450 and everything was clean except the top of the carb?? Why is that place so dirty? It wasn't really chunks of dirt but more oily black scum:excuseme:

Is it those small breather hoses letting fine dust in the top....

That area above the slide has air moving into it, and sometimes through it, but not much. It is "open" to the carb throat in the sense that it is not sealed off from it, and air is allowed in through a passage from the carb bore forward of the slide. That allows the slide to move freely up and down. When the hot start is opened air flows up into the area above the slide, then out the back through a passage that runs past the HS plunger and down int the intake behind the slide to add air to the idle mix. The vent tubes don't lead into this area, but run to the float bowl instead.

All of this results in the top of the carb being exposed to fuel vapors without having enough air or fuel run through it to keep it cleaned up, so it can get fairly foul looking. It usually isn't a problem though.

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