I have a 16.5 hp MTD lawn mower/tractor. All fluids are new and at proper levels. The battery is fine. For the first 20 minutes of use the mower functions just fine. After that, the engine will conk out unless I depress the clutch and disengage the mower deck. Running just the engine, I can run the tractor all day long. Once the mower is engaged, I get about 20 minutes of use before it starts to conk out. I know there are safety electrical lines. I wonder as the engine heats up that it might effect these lines. Please let me know if you have any insight to this problem.
Thanks,
Reed

Hi,
Two things that are the most likely, and nothing alike, are ignition armature failure and fuel filter clogged. Either can cause this because, as you correctly surmised, the engine develops more heat (ignition) but it also consumes more fuel to do that (filter.)
Peace,
Clarke