While I agree with all the good advice so far, the problem you've stated here is just the nature of the beast: off-road. There really is no way around the fact that some dirt just makes it through... especially if you are running in dusty conditions (i.e. non-watered tracks for big events or just non-watered tracks in general). Let me guess; you are taking a Q-tip, or other similar device, wiping it inside the carb after a race day and a small film of black is on the Q-tip? It doesn't look like there is any dirt there just looking at it, but after you wipe the Q-tip it'll show what has made it through the filter. This happens to me all the time. It won't happen on a watered, loamy track, but it will on the dusty ones. I'm sure there will be folks here that will say, "No, my technique is the best and dirt never makes it into my engine". Unfortunately, it does. All you can do is run a clean air filter that is well oiled every race day to catch most of the dirt.