Outdoor Power Equipment and Small Engines - Dies under load

I have a Sears Craftsman Trans-Axle Drive Eager-1 riding lawn mower. I have no idea about the history but it runs great at idle out of gear and on flat surfaces and declines while in gear. The problem is when I start an incline surface or engage the blades it bogs down and dies. To keep it from dying I have to hit the clutch quickly and move the throttle back down to a slow speed. When the mower is out of gear I can get the throttle very high but when in gear and/or under any load it will die at the same settings. Any information would be helpful as I have no idea about small engines.
Thanks much.

Post your mower's model numbers so we know what you have.
You may have a safety switch problem, which may be a switch
or a bare or loose wire. Once we know what you have, we can better help you.
Fish

This is the only number I can find on the engine 143-266022 ser-6149d

Waht horsepower engine do you have? Brand engine? It sounds like the engine is leaning out, needing more fuel when placed under a load.

You have a Tecumseh engine, and my guess as to your
problem is that you have a layer of water in the carb, which
stays there until you move, jostle, or go on an incline, then
some of this water gets "slurped" into the engine.
Remove the bowl nut off of the bottom of the carb, and drop
the bowl down and look inside.
Fish