MTD Briggs and Stratton Engine Won't Start?

My MTD model Briggs & Stratton engine 42A707 (700) won't start but keeps spinning. It stops running and will not run again. I replaced the fuel filter, drained off the fuel, and replaced the Magnetron unit. I have also replaced the spark plugs and cleaned the air filter. The choke opened and closed nicely, suddenly it cranked and started for 5 seconds and went dead.

It still cranks but just won't start. The battery is new. I noticed that I can crank the engine when I have the gear in forward or reverse without pressing the clutch. Any idea what happen here? Please help, thanks!

By pengteo from Redmond, WA