I was riding my bike the other day all of a sudden as I let go of the throttle it kept going. I killed the motor and started it back up and it immediately took off. I checked that there was no tension on the throttle but as long as the motor is on the rpm's are really high. It goes at about 60% speed when I'm doing nothing and there is no throttle tension. And when I pull back the throttle the rpm's are higher but it sounds really muffled and sounds like there is metal scraping together inside the engine. Anyone know what might be the problem/what I can do to fix it? Much appreciated.