If it is taking off when you start it then dying when you hit the brakes, it most likely has multiple issues, linkage and idle setting. First, do as Rolex mentioned, set the idle gap with the brakes applied, that way you wont completely close off the carb when you brake. Next, release the brake, make sure the carbs idle gap doesn't open up without you pulling the trigger. If it does check your linkage and/or the throttle trim on your radio.