Dirt bike Problem - Maybe Carb?

[Larz O]

I have a 1993 YZ 250 dirt bike. I rebuilt the bottom end not more than two riding hours ago, and it had a top end rebuild recently as well.

It starts up fine after two or three kicks, but after about 10 seconds it seems to get flooded our choked or something. It struggles and dies. After it dies, I can not start it no matter how many times I kick. I have to take the spark plug out and let the cylinder air out, plug it back in and then it repeats the whole process again. Also, the plug is abnormally oily when I take it out

I didn't mess with the carb during the rebuild, but is it possible that there is something wrong with it?

What other possible causes could there be?

[Elsa M]

have you checked your pistons