Im not MX expert (theres some great guys in tp who can give you better crit) but taking of general transport/motorsport shots:
In pic order
1. Good , I like that he's riding into the photograph but looks like he's riding it in a wheat field rather than a track
2. I dont mind the angle, but the bike is generally missing out of the photograph, its as much about the vehicle as it is, the rider
3. tilt is a little too much and too much of the bike missing
4. PERFECT.
5. Too much of the bike missing and a bit of his helmet too
To generalise, I think you need to not crop so much or photograph out too much of the bikes themselves. Get the whole thing in and rider if possible.
I had a quick check on your flickr stream as I wanted to know what focal lengths you were shooting, so looks like you were using a 70-200 or swapping lenses but there are so many BETTER images on there, than what you have posted.
I hope you dont mind me posting the file numbers as I prefer these so much more: IMG_0520, IMG_0721, IMG_0784, IMG_0189, IMG_0625 among a load of others.
Hope thats useful - you have some great action shots in the your stream