Husqvarna Lawn Tractor LGT 2654 (from Lowes)
B&S Intek V-Twin 725cc 26 gross hp
110 hrs.

I had a couple of problems with the 2654 in the past but they were electrical. Now the engine seems to lose power when mowing. It starts without any problem and runs fine until I engage the blades and start mowing. I am not talking about mowing thick tall grass either, just what I would consider normal (2-3" cut in a large field). I was able to mow really tall stuff when it was new without any problems nothing stopped it. I keep the oil changed and it has a new air filter. I have not changed the spark plugs but have cleaned them, they look fine. Any thoughts about what the problem is would be appreciated.

Sounds like its just running on one cylinder , probably wouldn't notice this until you put it under a load.
Start it and with a insulated pair of pliers pull one spark plug off and put back on if it doesn't matter when you pull plug wire that's the bad cylinder. Could be coil braking down after it gets warm.
LUCK

DG,
You are correct. After changing all the little things I opened up the valve cover and found a push rod dropped out. I checked the valve lash, 004" as required and I'm back up running strong. I don't have a clue how that could happen and am concerned it will occur again. Obviously both cylinders were not firing. A simple compression test would have isolated the problem. I will try to get a good used compression gage over the next few weeks for next time. Thanks for your help.

Probably vlave was sticking common . Put a little sea foam in your fuel , this will help with sticky valves .