W/ my 73 Toyota Celica, I'd occasionally take the air cleaner off and rev it while adding water in a super thin stream to the carb. The logic was that since water isn't compressible, it would flash to steam in the combustion chamber and break loose carbon that might be robbing performance in some fashion.
I haven't done it in years, but I wonder if this has any relationship?
BTW, the car ran great (because of or in spite of? I may never know) right up until the Sheriff confiscated it.