So I have a 2008 Spec B and the previous owner installed a lightweight flywheel with a stage 1 exedy clutch and pressure plate. The car has never been tuned or anything. Couple months ago, I felt the engine was jumping up and down in rpms whenever I press the throttle and just hold it at the same position. Now I feel it when I'm driving as well. Sometimes the engine just shuts down for less than a second as if it just stalled mid acceleration and then it bucks and accelerates back again. Suddenly, I got a code for my upstream O2 sensor that it was faulty so I changed the sensor and it helped for a little bit but now it falls back to the same issue as before. I've been digging my brain for quite a while to try and figure out what it could be and my conclusion keeps coming back to the flywheel being lighter than supposed to be and the ECU is freaking out trying to add fuel and air but can never reach a certain ratio as it isn't tuned. My current mpg average is 14mpg which is terrible and I have not been going into boost because those damn gas prices be too high at the moment If someone can chime in and let me know if indeed a lightweight flywheel does reduce mpgs, please let me know it would be greatly appreciated. As I said, I've been picking my brain for quite some time anything can help. Thanks in advance.
PS : the car is still bone stock except for that mod.