What causes high idling while in neutral?

Ok I replaced the cat converter with a cherry bomb muffler and cut my exhaust length to where the stock muffler used to begin I'm not the best welder so I have exhaust leak but I did keep the sensor intact in its original location my reasoning for doing that was the code read at advance auto told me that the cat was the problem so I cut it out and discovered that it was gutted so come to find out the spark plugs and wires was my fix as far as running and idling rough now it's back any suggestions besides getting the crappy welds fixed already planning on that