I was getting ready to store for the winter decided to do some maintenance first. Changed fuel filter, oil, cleaned carb, new air filter and plugs. The motor sat for a week when I cranked it up I noticed,it sounds like its missing at half throtle or better but it idles fine. It also hesitates when I throtle it up and somtimes with a slight backfire. Both plugs are gapped at .030 and firing. Any ideas would be appreciated.