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have a toro grandstand 60 inch deck it runs great until turning the deck on then it sounds and acts like it is losing half of its power i have checked coils changed plugs oil and oil filter in line fuel filter looks good has good gas in it has me stuck any help thanks!!!

Air filter? Underside of deck clean, blades sharp?

Ensure both spark plugs are firing:
With the engine off disconnect one spark plug then start it up...
If it runs same as before stop it, reconnect that spark plug and disconnect the other, then start it up.
If it runs same as before then this isn't the problem.
If in either case it won't run the side that is disconnected is the "good" cylinder and the side that is connected isn't firing.

Sounds like a blown head gasket on one cylinder, especially if you have spark on both cylinders.

Could be a bent push rod, push rod off.

Or it can be a dropped push rod or two...just depends on the engine model.