After running at full throttle for a few minutes, shut the motor down and remove the cover. Use your hand to compare cylinder head temperatures between the top and bottom cylinders....if it is running on both cylinders, the temperatures should be almost identical. If one cylinder is not contributing (cold ), check for spark and do a compression test. While the cover is removed, try running at full throttle again...these motors are prone to exhaust leaks and any exhaust leaking under the cover will contaminate the intake mixture and cause a low power situation. If it runs better without the cover, check for an exhaust leak.