The 'Stare'

I used to have that problem with karting shots. By the time the camera takes the shot the first kart has moved out of the focal range.
Try moving (if you can with the K-x) the focus dot towards the bottom and use burst mode, allow the camera to take 4 or 5 shots and it may have caught up with the action with one of them. Also I found the Sigma 70-200 a very good acquisition for this type of shot. Search for Llandow Kart Club on Flickr for some of my examples.

Thanks Steve for your advice. I will certainly try the thing with bottom focus point if I can select it. My biggest problem as I mentioned in yesterday or the day before's shot is that my SD card was only Class 6 and was only saving the first 2 shots I took of each burst before 'freezing' for just long enough to miss out on the odd really good shot! Next time I go i'll take a Class 10 card which should give me at least 3 shots possibly 4 before hitting the buffer. That on its own should make a good difference.

As it happens, my nephew is in perfect focus for this shot (blue /yellow helmet) and i was concentrating on following him round most of the time knowing that i'd 'pick up' a lot of the other karts in his races as i did so. Probably more luck than skill though as most of my shots were taken using 'centre weighted average'.