RE: 4 strokes
I agree with trying to adjust the low speed needle and I also strongly recommend putting in a new OS f glow plug if the engine does not respond favorably to the low speed needle adjustments.
I figure the glow plug installed in the engine now was probably new only a few tanks of fuel ago, but it has been my experience that new engines (especially four strokes) tend to eat the first couple of glow plugs pretty quickly.
I would not hesitate to install a new glow plug, you have nothing to lose as long as you don't throw anything away.