Won't start. Have changed plug and see that it is sparking by removing it from cylinder and cranking engine over. Removed air filter and cranked long enough that I know that the problem is not an air restriction. Have checked fuel filter and it isn't plugged. Also checked what I call a restrictor (cannot find a name in the owners manual) and it is clean and I blew air to be sure the lines were clear. Normally proper fuel/air mix, properly timed spark and compression is all the necessary ingredients. How would I check the spark timing? Could it have changed? I put a new main drive belt on, two new deck antischalp wheels and sharpened the blades. I have previously disarmed the seat switch, PTO switch and clutch switch and the presence of the spark plus engine cranking suggests to me that it is all working. Besides, this was a couple of years ago. I mention this to suggest that putting a new drive belt on would not likely dissrupt the wiring. Appreciate any help you can provide. Mike Rooney