I had a 1974 9.9 for a while. One thing that seemed to help was this suggestion from Leroy's Ramblings:
First off for the 9.9/15hp, if you are doing any work under the flywheel, you should consider replacing the original white or black friction ring on the timing plate (actually called the bearing). The new brown ones #322435 are made of Delrin and the word is from a experienced mechanic that this really helps these old motors as they seem to be a tighter fit. John Gill explained the later brown colored one was a tighter fit to make the magneto plate have less slop, especially at idle, so the pull of the flywheel magnets against the coil would not cause the timing to jump around as much and give smoother low speed running. The modified throttle cam was to try and give more throttle opening when accelerating from idle so the engine would clear out quicker as it tended to load up with fuel/oil after long periods of trolling. The revised ring also helped stabilize the magneto plate with the electric start option.
