Just solved the booming engine problem. Apparently, when the guy was sandblasting my exhaust manifold he dislodged the position of the thermostatic spring on the counterweight shaft. When I repositioned it correctly, I tested it as per the recommendations above and it began working perfectly. A road test confirmed that the boominess from the exhaust was gone and all is well. Thanks as always to my MoBros for steering me in the right direction. My slanty is happy again!