Had mower stored in a open but covered garage all winter. Started right up this summer but ran terrible, backfiring and rough. Put new plugs, new treated gas and gas line filter. Still very little improvement after running really well last year. It continued to backfire and would only stay running with choke pulled out part way and continued to backfire. It sat for a couple of weeks, started right up and ran better than ever, idle, part throttle , and full throttle. I thought it was a new motor. Mower was now outside and triple cover to keep dry. We had several inches of rain last week and today I uncovered it and had a very difficult time starting and it only ran half throttle and rough, wanting to stall every couple of minutes. After 1/2 hour of missing, running rough, it started to improve and finally ran perfect again. I'm thinking a moisture issue, but have no idea where to start, other than replacing the wires. Is this motor prone to issues? I thought this would be my go to horse, but now having second thoughts. Thanks in advance for help.