I have read that the FX has a dual ball bearing supported crankshaft, while the LA has a bushing supported crankshaft; and that they also have different carbs...the LA has air bleed type while the FX has twin needle type. I think the FX also has a metal backplate, and the LA has a molded composite material backplate.

The FX is sturdier, more durable, and has a better carb that provides more power with easier starting than the LA.