Hmmm ... 67 views and no comment? I'll give it a bash ... please feel free to disregard anything I might say (only opinions after all!)
In this particular position, you will have the shadow on your side of the bike. If you want to draw attention to the shadow I would suggest a different location where the shadow can fall on its own and be more distinctive ... or shoot just a fraction later so it gets clear of the kerbing.
If you want to focus more on the rider, I would suggest you stand on the other side of the rider (at the moment a fair bit of him is in shadow and you lose some contrast or "pop"). It may give a less distracting background?
I agree about using a lower shutter speed but for that to work you probably need to be at the side of the rider so that you can pan with the rider. That will require some practice as you get a slower and slower shutter speed.
If you look here:
http://www.thephotoforum.com/forum/photojournalism-sports-gallery/276586-dirt-karts.html
I shot these karts at 1/100 sec (well the two panning shots). I would start off around say 1/180 and work down to say 1/30 (but the keeper count is likely to get worse as you do ... but if you snag a pan at 1/30 you will have a "sweet, sweet" shot.
The other shot to think about is to get even lower and shoot more upwards - it will give you a more dynamic shot and less distractions in the background)
I would also increase the contrast here.
Given all that, please keep taking them - it looks like a lot of fun and I bet the bike rider loved having action shots taken.