80cc engine had been running fine for months and then all the sudden I couldn't accelerate at all. the engine starts fine, but no acceleration. The spark plug was caked with grease, so I changed it. It now accelerates, but will only get up to 15mph (no faster than I can pedal). I used to be able to get a top speed of 25-30. I took the carb apart and it looked clean. Noticed some oil near the muffler gasket, but the gasket looked ok and the muffler was clear. the engine sounds like it slows down when the throttle is fully open and at the top speed of 15mph. Any ideas on what is wrong and how I can improve speed and power back to how it was before? thanks