I was in the shop today and they told me I need to have the valves cleaned, shop time would be an hour for them to do it. Is this something I can do myself, I've never even heard of doing it. The mower is only a year old. Today it wouldn't start and I would get a backfire. Then eventually it started, I mowed a lawn with it and at the next lawn, backfire, it would turn but when I'd let go of the key, it would backfire. I brought it to the shop for that reason, and what do ya know, it started just fine so they couldn't do anything unless it was not starting there. Any thoughts on either of these issues?