Have you checked the carburetor to assure it is tight on the manifold? I know it sounds stupid, but it's a really common problem on slants for some reason. Remove the air cleaner and grab the carburetor with both hands and "twist it". See if there is any back and forth motion. Take note of where it is. It's not always the two manifold attaching bolts. Sometimes it's the screws that actually hold the carburetor together. They can all work loose over time and cause a vacuum leak making the engine run badly.
