Yesterday I was mowing and my friend was using my walk behind. He turned it off because some guy needed to borrow gas and then he couldn't get it to turn back on. We spent a long time trying to get it to fire, and it finally did about 20 minutes later. It started really slowly and finally putted up to rpm. Later that day I used it again and I could just BARELY get it to start. It took me about 5 full minutes to get it running. It would try to fire and turn over for a few seconds and die. I was finally about to get it started and I used it for that lawn. It sounded like it was running decent but would blow puffs of smoke every now and then. It also didn't cut well at all (lower rpms perhaps). I tried starting it later with no luck, I just can't get it to fire. It will turn over for 4 seconds at best and die. It has a brand new air filter and oiled prefilter, and plenty of oil (just changed last week). Any ideas of what I can check etc? How much will a new engine run me if I need to get it. Oh yeah, its a B&S I/C 10.5hp. Can it be rebuilt? Thanks.