I have a 1980 Toyota corolla with a weber downdraft carb installed by previous owner. My problem is that when you shut it off it runs for a few seconds and will either A. Diesel, rev up and die. B. Backfire loudly and shoot flames. C. Backfires but sounds more like a burp. It also idles rather high. Any ideas? I've had two carb tuners look at it and they did their best but to no success.