Header flanges

[zimmy]

I have a Contemporary from 1991. It currently has Webers on a bored 427. I have recently tuned the carbs correctly. They now run at the proper A/F ratios and I have returned to a problem that has plagued the car with Webers from the start.

Whenever you get off the throttle to shift or just cruise or returning to idle, there is a tremendous amount of exhaust backfire from the lean condition that is created. The backfiring is caused by the lean fuel mixture hitting the hot header with fresh air available. The air is coming from an exhaust leak of some sort. My slip fit connections are sealed with band clamps that I believe are also too far downstream to affect the popping. I recently replaced the exhaust gaskets with raised edge copper ones to no avail.

I discussed this problem with a skilled fabricator that I know that works a great deal with Cobras and Daytona Coupes, etc. She told me that she recently did a repair for this same problem on another Contemporary. Her discussion was about the header flanges on the Contemporary supplied header tubes. When the were welded to the pipes, they were never flat faced. Almost all of the ones that she worked on were cupped and uneven due to the welding. She removed them from the car and flattened the faces of the flanges on a belt sander until they were completely flat and mated up perfectly with the cylinder head ports. She said that the popping went away completely.

Has anyone else experienced this problem and found the same solution?? My situation is getting bad as the car sounds like World War 3 whenever I get off the throttle. I don't mind it much but the rest of the world (my wife particularly) doesn't enjoy it. Any thoughts or comments would be appreciated. Thanks.