Depending on old that Holley is, they say the carbs built in the last 6 years have backfire protection built in. Inline check valves now save the holley of today.
http://www.holley.com/HiOctn/TechSer...fo/CI-121.html
"Many of the new Holley performance carburetors built today now incorporate "power valve blow-out protection". With this provision, the power valve diaphragm is protected from damage due to engine backfire, by a check valve that is located in the throttle body. This check valve is designed to be normally open but quickly seals to close off the internal vacuum passage when a backfire occurs. Once closed, the check valve interrupts the pressure wave generated by the backfire, thus protecting the
power valve diaphragm."
Tim
Actually David pick any on the links off the bottom of this page they all touch on what I asked about
http://www.holley.com/HiOctn/TechSer...nfo/CI-44.html