Small Engines (Lawn Mowers, etc.): Toro Lawn Tractor stalls under load, toro wheelhorse, piston rings

Question
I have a Toro Wheelhorse that runs fine on short grass or level ground but really struggles when the grass is relatively high (not overgrown) or when going up a slight incline.  Replaced the air filter, fuel, and fuel filter. It does have a fuel pump.

Thanks,
Eric

Answer
Hi Eric,

If the engine is old, the piston rings may be worn.  A compression test would confirm that.  
Another possibility would be a lack of fuel flow.  Try opening the carb's high speed mixture screw a bit.

Let me know how you make out.
Michael