It sounds like you now have a vacuum leak. I would recommend changing the UIM gaskets, they fail easily when they get old. Look for other sources you may have disturbed while doing the work like vacuum lines behind the engine.
If low end power is also affected make sure the IMRC is closing.
I have no idea wat the hell the deal is?? I would like to know myself. I have a muffler but no dual exhaust on my tour... so i dont think its your exhaust.