I have a Craftsman Model 917,272751,18 hp lawn tractor. Problem: the engine shuts down and will not restart when I move the throttle to the slow position prior to emptying the grass containers. It also runs a little rough and backfires occasionally when moving the throttle from slow to the run position. I must lift the hood and move the tractor into the shade for 15-30 minutes before it will restart. The tractor has run well for years, this is a recent issue. I have changed the spark plug. When the engine is first started (cold) started, I can move the throttle to the slow position and the engine will run normally. Thanks for your help, Larry

It seems like the valves are not working too well, backfire indicates that the valve is not closing properly and the fuel is getting pushed past the the valve and igniting in the hot exahaust pipe. It is possible that this tracktor is overdue for an oil change, an oil change would prevent damage to the valves and help cool the engine. I think your problem could be due to low oil level either currently or in the recent past.

As you explained it runs normally on a cold start and then gives troubles as it gets hotter, I think it is overheating and new oil will help keep it cool and get it running smoothly. However it is possible that the valves are damaged slightly, this is confirmed if the tracktor is low on power.

I think it might also be possible that the carberator is a bit stuck/gummy. Clean out the carberator and airfilter.

Take care!
damnor01