Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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This sounds like the engine may only be running on one cylinder. It sounds good until you put a load on it and then it looses power.
I would check the push rods on both cylinders to see if you have bent a push rod. You can easily get to them by removing the valve covers.
It is possible that you may have sucked something into the carburetor that might cause you a lack of power but this would normally show up all the time not just when you engage the mower deck. However I would not completely rule it out, it is just not very likely since it would have to pass through the fuel filter, the fuel pump and then into the carburetor in order to cause you any problems.