If it seems to run fine for the first minute or two, and then slows down, it is probably overheating from being too lean. It may only be a couple of clicks worth, but that is enough to do it.

Richen it up just a tad and try it again.

The FX should have more power than the LA. If you are trying to run them synced, the settings will be different for each.

For engines of different types, I would suggest syncing by rpm, and not by sound.