newish Briggs backfires and won't start
I've had some issues with an older Craftsman riding mower. (http://forums2.gardenweb.com/forums/load/tractor/msg0413270418571.html) I installed a new motor this spring and it worked fine all summer until now. I don't have the exact model number in front of me, but it is a 13.5HP Intek, series 21B800.
A couple of days ago, I had mowed for 20 minutes or so and the mower slowed to a rough idle. I thought I had run out of gas, so I stopped and turned off the key. There was still some gas in the tank, but I refilled it anyway. Struggled to restart, and it never got going better than the rough idle, and even that for less than a minute. Now it won't start at all, but occasionally backfires pretty strongly. Here's what I've done:
drained and refilled gas
replaced gas lines
removed and cleaned carb
replaced overfire solenoid with plug (I don't think that was teh problem, but wanted to rule out the possibility)
replaced spark plug
checked spark with spark tester
It seems to have good spark, and gas seems to be getting to and thru the carb - in fact, the spark plug is soaked and it seems like gas is getting all the way to the muffler, where it sometimes ignites. I pulled the valve cover, and the top valve goes in and out when I turn the flywheel by hand, but the bottom valve does not. I can push in the bottom valve by hand, but the push rod does not move in and out at all when I rotate the flywheel.
What should I be looking at to try next?