first of all, are you mowing 12" high, wet kentucky blue grass?

if not, then are you sure both your cylinders are firing?... when you lubed the spindles on the deck, did they spin freely?...maybe your governor is not functioning properly, thus not throttling up enough under load?

at 50 hours, maybe it's still under warranty?

"..are you sure both your cylinders are firing?"

That's kind of my thought. One might be weak.
Did both of the old spark plugs appear the same as to color?

Maybe check the air filter?

Kevin

It's possibly starving for fuel. If it has an O.E.M. inline fuel filter, you might try replacing it with an automotive filter so you get greater capacity for storing the stuff that a fuel filter catches before becoming "stopped up". I like the clear plastic kind so you can see if it has trapped water. I think you probably have 1/4" fuel hose so getting a nice, robust filter at a local parts house or even Wal-Mart is no trouble. Pay attention to the directional arrow pointing the right way to install it. Don't worry if you see a pocket of air in the top of the filter that won't go away, that's normal and does not mean your are sucking air or anything. The reason I suggest it may not be getting enough fuel is that you say it starts well and runs well until you try to cut grass. If it isn't acting strangely otherwise, it might not be getting sufficient fuel to meet the engine's needs when the real load is handed to it. Go out there and try to mow some grass the way you did before. Do this for about 15 minutes and then cut off the engine right there (so you can read the evidence on the spark plug tips without running the engine without being loaded) Let it cool down a bit and remove the spark plugs to inspect the tips. If the tips are dark black and sooty looking, you are getting too much fuel and/or not enough air. In that case start looking for a restriction in the air induction system (air filter and check any ducting leading to the air cleaner for foreign material, like a mouse nest). If the tips of plugs are white in appearance, you are not getting enough fuel and the engine is running too lean.

And what about purging the tranny (if it's a hydro)? Seems to be alot of talk about the need for that on a new one (haven't owned a hydro myself, yet). Also, remember the problems a couple years back with a certain Craftsman model (same mfr?) that would have sticking/locking brakes? It was happening to a lot of people but the fix was simple like a squirt of WD-40 or some such thing like that. Otherwise, check for slipping drive belt or siezed deck pulley.

Bill

Unplug one sparkplug wire then start the engine, then try
it the other way around. See if that gives you different results. Let us know, and also give us some of the engine's model numbers.
Fish

I told you that I lost 50% power when the deck was engaged; no wonder this happened, as only one cylinder was functioning; thus only half of my 22hp was trying to spin the 54 inch deck & mow grass. The right cylinder is not functioning. I pull the plug wire and the engine does not lose one (1) rpm. Pulling the plug wire off the left cylinder immediately kills the engine. I have hot spark to the left plug; hot spark coming out of the left plug, good compression from the plug hole; yet no function & no power. I think a valve problem, maybe just backed off.

I do not have a service manual, so wondering how much of a job it would be.

You said: "The right cylinder is not functioning."

Then you said: "I have hot spark to the left plug; hot spark coming out of the left plug, good compression from the plug hole; yet no function & no power"

So which is it?... left or right?

if you checked the spark on the "working" cylinder, then no S@#$t it's OK... how about the right cyl.? does it have spark and compression?....

if you had leftie-rightie problems when typing (just kidding), then perhaps you are not getting fuel in there.... maybe your intake valve or exhaust valve is not actuating.... is the plug clean or fouled?....

"and also give us some of the engine's model numbers.
Fish"

"I do not have a service manual, so wondering how much of a job it would be."

What brand engine?

Bill,
Glad you caught my lefty/righty mistake, just another sign of old age setting in.

It was the right bank not functioning; completely dead even though good spark & compression. Figured it had to be a valve problem, and removal of the valve cover revealed a push rod completely off. I checked it for straightness and reinstalled.
I now have 22 horses eating grass!!!

Thanks to all for your input.
KyBill