I worked on one for a neighbor that had the same symptoms. After going into the carb three times and checking a bunch of other stuff, I replaced the new Champion spark plugs with NGK spark plugs (just because I had some) and it magically ran perfect! Don’t ask me why, but others here said they had experienced this too. Since then, I’ve used only NGKs in OMC 9.9s and 15s.
Dave
