Why is a 2 stroke dirt bike faster than a 4 stroke dirt bike?

In short the design and the inner working of a 2-stroke vs a 4-stroke allows a 2-stroke of a similar size/weight to make a lot more power than the equally size/weight 4-stroke. The downside of this is that it's a lot more polluting and a lot less energy efficient.

The reason they are faster than a 4 stroke is because the 2 stroke is for racing. The gas and oil are mixed together to make the ultimate engine for your MX. A 4 Stroke is mainly for Trail Riding in my opinion.

^^ this is only true for little bikes like 85s and 105s...

In real bikes 2 strokes are still faster CC for CC, they are lighter and the engines are more efficient for making power (fewer moving parts).

HOWEVER, the AMA has allowed for 4 strokes over twice the displacement to race against 2 strokes, for example a 450 4 stroke vs a 250 2 stroke, in this case the 2 stroke has no chance.

If the AMA were to equal classes, and the 450 were against a similar sized machine like a 500, the 2 stroke would dominate like the old days!