Small Engine Troubleshooting Problems? Ask a Mechanic Now.
run the mower and pull one spark plug wire at a time to make sure both sides are firing. If you are losing power only when deck engaged, you are losing power to one side or the other. If mower engine continue to run on each cylinder singlely. I would pull valve covers and check the valve clearance. make sure both push rods are in good condition and gapped correctly
I hope you got issue resolved.
Please remember to RATE me as I strive to be the BEST and your ratings help me to achieve this goal!