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.

[rokndad]

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.

I'm sort of new to the Weber game, but have worked on enough carbs over the last 45 years to understand this a bit. I just installed a very similar set up on my BB FE, and have no leaks whatsoever on the exhaust flanges at the heads, and the slip joints are sealed with no leaks.

The Webers don't want to run without backfiring on decel or slight accel, without being 1 1/2 turns on the idle mixture screws. The pipes pop like hell with only turning each in 1/8 turn!! Put it back to 1 1/2 turns, no popping at all. Idles and runs rich, with dark charcoal plugs up to 3000 rpm or so then leans out with the 145 main jets. I'm going to up them to about 155 in the future to see how the car likes it.

I don't know how you can get the FE to run lean at low rpms and not pop on decel. I haven't found a way yet. By the way, initial timing at 16 degrees, 38 all in at 2500 rpm. Tried initial at 12 and it wanted more mixture so I upped the initial. Runs like a M-F'er all the way up through 6000 rpm so far.

[mickmate]

I have a CCX in the shop now that has had the exhaust flanges faced flat. They are now only .230" thick and all bent like a banana and are blowing by even the MrGasket copper you mentioned. They are getting my new 3/8 flanges right now.

[h2oman]

Had the same backfire problem until I got rid of my slip fit pipes.

After installing the flanges no more backfire. Installing Exhaust Flanges

Good Luck

[csx wnab]

you need to look at this web site. you will then be calling them to purchase fe gaskets like i did. www.remflex.com. i even had them make a custom gasket for the 4-hole flange that superformance uses. even has it's own part number.