Bill could be right on the money that you may losing spark. Safety switch, funky kill wire, spark plug or coil shutting it down and the fuel doesn't get burned off the plug. Either whatever cools off enough or the thing that is shorting the coil gets bumped by the time you check for spark. You may try unhooking the the kill wire that goes to the coil and running it in place for a test to see if it will occur then. These carbs also have noted float valve issues. It would be worth a check your oil level to ensure that fuel hasn't overflowed into the crankcase.

Change out your spark plug. Have seen similar issues with Kawasaki engines. Would run great until you put it under load (engage the clutch) and then it would stall. Seems like it's way too simple but I've cleaned carb's, reset valve lash, and checked flywheel keys on machoines that just wanted a fresh plug. Good Luck P.S. Make sure the new plug is gapped correctly.

Check the safety switch under the seat. Mayb e it is being tripped when you start driving and going over bumps.

I'm having the same problem. The first thing I checked was the deck pulleys. Sure enough I had an idler pulley that was locked up. Got a new one cleaned the underside of the deck installed the belt fired it up pulled the PTO and down the engine went. It would still run but at less than optimal speed and if you actually tried to cut grass it would pull the engine down until it stalled. I put it back into the garage and started with the fuel system. Replaced the filter and checked for good output from the pump, it was very good. Pulled the plate off the top of the carb to check for foreign matter in any of the fuel journals, zip. I pulled both plugs and the left on was fairly black though not oily and caked with carbon. Replaced that plug and checked for good spark-none. Bad coil pack? Got a new on and used emery cloth on the flywheel. Still no spark at the plug. However if while the engine is running I short out the plug lead the engine slows down even more and runs just like you would expect with a plug shorted out. I still don't understand why there is no spark in the plug gap. I have observed the engine running looking for an errant spark in the wiring. Don't see any. Any body got a god idea what to do next? BTW the compression in that cylinder is 150. Now what?