Older Craftsman 15.5HP rider. B&S 31C707-0230E1 engine. Some history: For some time now, the mower has not spun up correctly when starting, like it has a weak battery connection or bad starter. After mowing for about an hour the other day, I stopped for about 10 minutes. When I went to start the mower, it would not start, and it was backfiring through the exhaust. While it has had the poor spin up problem for a while, it has always started. I read up on this condition and decided it needed the valves adjusted. Got that done. The spin up problem has gone away, but the mower still will not start and still backfires. I checked and I have spark at the plug. Checked the flywheel key, it looked ok, but I put in a new one anyway. Still no start. No fire. Spins up fine. Still backfires through the exhaust.