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I have a Troybilt Pony 17.5 HP. The mower starts and runs great but once I start mowing the grass sometimes the engine will start running rough. It will not smooth out until I disengage the blades or throttle it up and down. Does anyone have any idea for this?

And which engine by model number are you having problems with. We can't tell you what to look for without knowing what engine you have.