I recently acquired a John Deere 102 Briggs and Stratton 17 HP OHV. Its pretty new and in great condition, but when I got it, the engine did not run. I replaced the battery and spark plug, checked the oil and air filters, and cleaned the carb. It now starts perfectly at full throttle/choke and runs fine. However, when I reduce the speed to mid throttle the engine starts to die. Any ideas?