ok i blew my intake gasket on my 305 so i put a performer intake on it i have a 650 holley carb with a spacer. now it will shut off after about three miles any ideas?????? it will sit there and idle all day it only when im driving. no air leaks.so im at a loss

 

If your timing is not set right, or your idle is too lean, it can build too much heat in there, causing something like a sharp edge, or a carbon build-up, or even part of the spark plug, to get hot enough that it keeps igniting without the regular spark. This is called dieseling or run-on. I'm not sure this is what you have, but this is possible.