Yesterday I started winterizing all of our power equipment. The riding mower would not start. (Briggs, 12.5 HP, I/C, 15 years old). It had spark. I took the carb off and cleaned it from inside out. Still wouldn't start. Sprayed starter fluid into the carb. It ignited and backfired thru the carb! I thought timing problem. So, here's my question, can single cylinder engines be timed? If so, how is it done? The other thought I was thinking is that maybe the intake valve was hanging up...but when I had the carb off and turned the engine over, I saw that the valve moved back and forth. I suppose maybe the valve could be carboned up and not fully seating. What do you think?