It's mainly an issue with throttle linkage setup. Have you read through the Losi 8 thread?

http://www.rcuniverse.com/forum/m_4762247/tm.htm

I read every page of that thread and read through the parts about the throttle multiple times. Search for "throttle" in there. There's photos and everything on how set it up correctly.

Losi has also released an addendum concerning throttle setup.
http://www.teamlosi.com/ProdInfo/Fil...ndum_LoRes.pdf

Keep in mind that this setup requires very little throttle movement to engage the brakes, so set your EPA for the brakes LOW. Mine is only set at about 30 at the most. And certainly make sure your servo isn't trying to pull the carb open past it's stopping point as well.