hey,my vp ss 92’ 304 efi commodore has just started to backfire at low reves , but above 3 grand it stops and runs fine, i changed all leads an plugs but i have no idea hope someone can help.skip

My first guess is a timing issue.

Are you absolutely SURE you got the plug wires on right?

Find out which one is causing backfire: Disconnect, and ground (lay the spark plug wire end on the engine block) each spark plug wire. Take note of which one(s) no longer causes backfire when disconnected.