1972 911T MFI - Runs fine when cold, rough when warm. Why?

[M491]

My car is a 1972 911T. Does anyone know what would cause MFI to become rough, stutter, and backfire loudly whan it warms up? I have the hoses hooked up from the heat exchanger to the thermostat, as well as the one from the fan housing to force air through the heat exchanger. The car runs great, idles smoothly, etc.until it warms up. Then it becomes undriveable, really. The neighbors all throw themselves to the ground, thinking there is a sniper shooting at them. I mean, the backfire is so loud I think I lost some hearing yesterday! Any advice would be highly appreciated. I have the CMA manual. Thanks.

[edmayo]

There can be a hundred reasons why, but it sounds as if you're running too lean which is why it runs well when cold, the enrichment is on, but poorly when hot and leaned out. When did this start, was any work performed prior to this occurence. Did it ever run correctly (well we know it did when new) How long have you owned this car, do you know any maintenance history. Has it been sitting for a long time? Distant diagnosis is always risky at best, so often I'll see something that no one thought to mention, that's why the more history the better.

[RickS]

Another thought might be your plug wires. If they haven't been replaced in a while they build resistance when warm and can really mess things up. I had similar symptoms and the wires cured them. But like Ed said, there can be many contubuting factors.