hard time with my 86 stang

trying to upgrade to a 65mm throttlebody and having idle problems with some back firing and rpm fluctuating any suggestions

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Well, if that's the only change you're making... that's the problem.
That's a speed density ECM and touchy to set up correctly becuase it changes airflow per throttle angle.
You have to get the engine warmed then set up your base idle to about 550 rpm with the ISC disconnected and ports closed. Then you have to set the TPS to .95 - .99 volts
If that's the only change you're doing, I think you'll find it's a waste of effort.
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