Is this a stock engine? If it is 13:" is pretty low. My first check, as others have said, check vacuum lines. I take a can of carb cleaner and spray a small amount on Vacuum lines, base of carb and vacuum fittings. If that all checks out, check plug wires. Are you running a HEI distributor or the stock cap rotor and points? If you are running points you may need to check the dwell.