I have 2 each of the Magnum .52XLS, OS .46AX and GMS .47. All of them have run excellent for me. The OSs were easy to adjust pretty much right from the start. The Magnums took a while for me to be able to get a good idle & transition. The GMS took even longer, maybe a gallon or so.

The Magnum and GMS engines had the front needle valve. I have never had much luck with the rear needle except of the AX. The AXs and Magnums are on twins and the GMSs will be on my next twin project. I would not put an unreliable engine on a twin.

I skipped the FX, by the way. My SF peeled and friends' FXs peeled so that's when I went to Magnum. Now that the liner is supposed to be fixed, I gave the AX a try. I think the later FXs were good, too.