I'm new to the business and using an older Craftsman 42" riding tractor with a Briggs and Stratton 20hp engine, Model No. 917.270940, until I can afford a new mower. I can crank it when it's cold and it runs OK; misses every now and then, about once every 15-30 seconds. After about 3-4 minutes, it shuts down like you just turned it off. It will not crank at all after it has warmed up. I recently tuned it up with new spark plugs, air filter fuel filter and oil change. Can't find a manual for it online for troubleshooting. Any helpful info would be greatly appreciated. Thanks!