Here are some of the differences between an FS and FX engine.
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But basically the air/fuel mixture is different between the FS and FX with the increase in incoming air flow resistance caused by the Donaldson two stage air filtration system. There are several other parts involved that are different which make the change over difficult.
A Donaldson system can be cobbled onto any engine, but the end results can both look and perform poorly.
I priced the parts needed to make an FS730 to an FX730 for a customer, it was over a grand and some of the parts had six to eight month delivery!!! Eventually I did repower his machine with a new FX730V which was shipped to me in four days. He was and still is very happy with his repowered machine.