I had a play around with it and it was a bit of fun, although it definitely needs work. It's a really nice concept. In the custom stage the background suddenly turns black and it lags. It needs sound effects, a main menu and different car colours and weights would be nice. First of all though removing the black screen problem is a must. As I said nice concept, just needs to be polished and expanded upon. Good job.
There is a problem where if you type in the name of an other player you can play as them. So you could improve someone elses score and they might not like that. Maybe instead your name is the kongregate username and that would advoid people playing as another person.