I have a Traxxas 2.5 Engine and every time i start it it will run fine for about 20 seconds and then shut off please help

Reset it back to factory settings (HSN, 4 turns from closed and LSN 1 1/2 turns from closed and the idle gap should be .7mm to 1mm)

If this still doesn't work, check to make sure that you have no leaks in the tank, lines or the engine.
If that is ok and still not working, maybe that the engine pinch / compression is gone.

NOTE: Please post more information in the future, so the question is not so general. Post more infomation about what you have checked and tried.