If everything else fails to fix the problem, you might try cutting the remote needle valve off of the backplate and mounting it separately.

The FX engines with the needle valve cast into the backplate were notorious for allowing the engine heat to transfer directly to the needle valve, which altered the mixture setting. The 91FX was one of the worst cases, which OS retrofitted with a different style needle valve that used a plastic housing to stop the heat transfer. I've seen at least two of the 46FX engines have the same problem. In both cases, cutting the needle valve off of the backplate and eliminating the heat transfer cured the problem.