If you haven't touched it, or know where it is, the low speed mixture screw is probably where you need to focus your attention...for now anyway.
Look in the hole in the center of the carb throttle arm...these's a small brass slotted screw that more than likely needs to be turned clockwise to lean the idle / mid range fuel mixture. Most (if not all ) of the FX's I've dealt with (lots of 'em ) are set way rich from the factory.
Try turning that in (leaning ) clockwise about 1/4 turn and see if that helps. If it makes things worse, then go the other way...

If that helps, then make your adjustments in 1/8 turn increments to fine tune it...and you will probably need to re-adjust the high speed needle valve too.

And a good, fresh glow plug might help...O.S. #8, or A3 in colder weather works well for me.

If you do what's called the "pinch test" you can narrow down if it's too rich or too lean.
Pinch the fuel line for a few seconds while the engine is running at a fast idle (careful of the prop ) and listen if the engine speeds up or just dies.
If it speeds up a lot before quitting then it's too rich. If it just quits, it's too lean.