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911 1972 MFI problems

Fuel System

Vehicle Information: Model: 911T; Year: 1972; Body Type: Targa; Engine (type, size, modifications): 2.4L; Total Mileage: 87000;

I recently bought a 1972 911t with MFI. The car is a little cold natured and backfires often on deceleration but otherwise runs well. The sparkplugs are typically black with soot(not oil) but not fouled. A previous owner replaced the stock heat exchangers with stainless but unfortunately bought ones without the tube needed to attach the warmup hose to the thermostat on the MFI. In a previous question, I was told that this hose is required for normal MFI function. Since then, I had a equivalent tube welded into the left-side exchanger and installed the warmup hose to the MFI. Now the car runs terrible below 3500 RPM but smooths out at higher RPM and backfires much more. The thermostat is indeed getting heat as it is hot to the touch. If I let the car cool down and remove the hose, I am back to the original performance. I assume from all of this that I now need to adjust the MFI system with the warmup hose in place. Is this something that I can attempt myself or should I seek professional help? I've accumulated quite a bit of adjustment info from the Pelical website and others.

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