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Discussion Starter #1
Have a question on my '78 Lincoln Mark V. Car developed a backfire at idle (reving and letting accelerator go) and upon downshifting down hills. Was wondering if it's a problem with the ethanol gas additive here in the N.E. Have rebuilt the 4bb carb and reset timing. It backfires retarded and / or advanced. Also checked all vacuum lines and plugs. It has electronic ignition. Otherwise car runs great and has tons of pep. Realize it's not a "Corvette" question, but may be a generic problem. Any thoughts?
 

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There are several possible causes.

Unburned fuel getting into the exhaust and then firing because you may be too rich.
Carbon build up for the same reason.
A hole in the exhaust manifold or a split gasket.
The emmissions system out of adjustment.
 

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Discussion Starter #3
Always Wave - Thanks - will check and lean out the carb tomorrow. I am currently 1 1/2 turns out from rough idle.
 

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Jon Schapiro said:
Always Wave - Thanks - will check and lean out the carb tomorrow. I am currently 1 1/2 turns out from rough idle.
Use a vacuum gauge.
Start with the screws 1 1/2 turns out from fully seated. Turn each equal amounts for highest vacuum reading.

Make sure that the idle speed adjust is not so far that the throtle blades expose more than 040" of the transfer slots. That fools a lot of people.
 

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in addition to the above check for a possible leak in your exhaust system. I had a pin hole in my passenger side collector that I didn't notice until I installed the headers and it caused some popping as well.
 

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Discussion Starter #6
Think I got it about 98% solved. What is interesting is that leaning it out (turning idle screws in) increased the backfiring in the mufflers. Would have thought enrichining it would do that due to excess fuel in A/F mixture. Thanks for your help!
 

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Burnt Exhaust Valve?

I use to work in the shipyards here in Houston, and one of my jobs was to maintain the trucks. We had a 1 ton truck that developed the same symptoms as what you are describing, and we found that it had a burnt exhaust valve.

Hopefully your problem does not turn out to be this, but you might do a compression check to be sure.

Bill

http://home.earthlink.net/~bigdogvettes/index.html
 

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You see stock cars going into the corners backfiring through the exhaust all the time and it is caused by gas being sucked out of the transfer slots . The throttles are open too far and the transfer slots are exposed. You need to close the throttle plates more.
This is the reason why motors with a decent cam need the throttle plates drilled so they can be closed at idle covering the transfer slots.
 

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Since I gutted my cats my 96 LT4 will backfire like a mother fugger if I bang the gears and then take my foot off it at high RPM.

It'll scare the crap out of a neighborhood.
 
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