I was riding home tonight when I had to ride over some rough pavement. I didn't think it was unusually bad, but immediatly my engine had no power to accelerate. It idles smooth and starts fine, but when I try to pull away from a stop sign I get little help from the engine. If I open the throttle about halfway the engine revs and power drop and I can hear a grinding noise coming from the engine (but only when the throttle is opened). When I close the throttle the bike gets a little bit more power as the throttle closes (less than 1/4 open) and when the throttle is completely closed (after having just been opened) the engine will backfire. The way I got home was by opening the throttle about halfway and then slowly rolling off until I found the point were the engine would slowly build up power, which was about 1/4 open or less.
Is this a fuel/air mixing problem? A valve problem? Timing problem?
Please help me out here cause I'm at a loss.