reason It's shutting off is because the return spring on the carby is pulling the throttle closed, you're gonna want it open enough so when you pull the brake it doesnt shut off, it shouldnt even idle lower, if you hae it set right. but you dont want it idling so high that when its sitting there idling it wants to run off, it should sit still.
Atleast thats what I think, if anyone reads this and it's wrong..uhh...Tell me
Good luck!
~Wendell~