Engine Husqvarna Rider 155 Engine Stalls

I have a 155 rider with a 15 hp Kohler command engine. It is a great mower about 9 years old.

However the engine will not pull a load. For example, when I try to raise the deck, or move at any sustained speed the engine stalls down and looses power. When I remove the load it returns to normal speed and sound .

I would appreciate any suggestions, I have replaced all filters and it has a new battery and spark plug.

Engine Husqvarna Rider 155 Engine Stalls