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83 911SC runs rich when cold

Engine

Vehicle Information: Year: 1983; Body Type: Targa;

Hello,I recently removed my cat and stock muffler in favor of a bypass (test pipe) and Dansk sport muffler on my 83 SC. The test pipe had a bung for the O2 sensor so I kept the sensor on. Since install, I've confirmed the car runs richer than it did when the cat was on. It remains rich far into the warm up cycle - up to when the oil temp needle just passes the first white tick. On cold days this can take up to 30 minutes or more to reach. Once warm the mixture adjusts within spec and runs great.My gas millage is suffering as a result and was wondering how to diagnose. Could it be the warm up regulator? Cold start valve? The car cranks and starts with no problem.Another issue (possibly unrelated) is the car now pops mini backfires on hard deceleration but I attribute this to less back pressure in the exhaust system. Let me know if this need to be checked as well. I don't think I have any exhaust leaks.Thanks.

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