The last few days I have been trying out my new to me XR650R. What a fun bike! It's a 2005 and has been used less that 10 hours. The original owner had the local stealership uncork it and install a Pro Circuit T4 slip on. I'm not sure why but they left the smog pump in place. The bike seems to run fine but when you let off the gas it really pops like it is backfiring. The noise is much louder than when you are accelerating. Is this normal with an aftermarket exaust? Or do I need to work on the carb?