FX and it's son the AX use ball bearings to support the crankshaft, where the LA series uses bronze bushings. The ball bearings reduce friction, allowing the engine to spin faster and make more power. However, they are heavier than the same displacement LA engine. The LA engines are also smaller sizewise. The carbs are different, too, the LA uses an air bleed carb where the FX and AX use a two needle carb which is more efficent.

Andy