Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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The first thing that I would look at is the transmission drive belt from the engine back to the transmission. If it hasn't been replaced it is most likely stretched out and slipping which would cause your problem especially when on an incline or put under a load like that. The belt itself may look fine but could be stretched out just enough that it allows for it to slip when you put the machine in a pull or go up an incline.