I may be totally off base here, but I'd recommend you trace the wires that run up along the right side of the engine and look for a misrounted/pinched wire. It's happened more than once that working on the right side of the engine results in a pinched neutral wire causing the problem you describe. Especially when replacing the sprocket cover after changing the front sprocket. Done anything on the right side of the engine? Or anything at all around the engine lately?
