Missing at idle

[Derek.Matirne]

Maybe someone on hear has had this problem, and can help me..... 350 with new HEI, misses when at idle.... It did this before I replaced it... Timing light shows some of the plugs missing on occasion, not constantly.... I have not replaced the plugs or wires, because the both look good... Also I am running open exhaust just in front of the rear wheels... Could lack of back pressure cause this? When I give it gas, it runs great, just Disney doing great at idle....