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?