Air Induction System blockoff plate or make your own by making sure the tube is airtight. There's a little oring under that plate with the tube coming off it, no air leaks. There is a tee connection on the top of the carb that provides vacum for this and a vacum oprated fuel petcock, make sure that is sealed too. All got modded with the IMS tank probabily. It caused backfiring and it's thought to contribute to cherry red exhaust syndrome so people removed it.