Grap hold of the flywheel and give it a good side to side shake to see if you notice any movement of the crank shaft. Your word useage and description sound so familiar of one of the guys that had the main bearings going out of his Bat. The leaning out and acting like it's out of gas, jumpy, fast and slow for no reason, is sadly the same descriptions he used. Please prove me wrong because in this situation I hope I am wrong.
Lamborn's Miniature Engines