When trouble shooting my Fox .049 sport (pre-64) we found the spraybar had a really small orifice. It measured about .017" dia.
A Cox .049 reed engine spraybar measured .040" in comparison, and a Cub .049 spraybar orifice measured .025".
We had damaged the original spraybar removing it, so we drilled out the replacement spraybar orifice to .025" (same as the Cub). The Fox had refused to richen up when we first ran it, even with the needle all the way out. My friend has the same engine, and it ran almost identical to mine.
After installing the new spraybar, and sealing both sides with J-B Weld (to be sure there were no air leaks), it ran perfectly! We have not modified my friends engine like this, but suspect it will cure his run problems as well.
I realize that this could also have been caused by air leaks, but the small orifice on a relatively hot engine (for the period when it was made) just didn't make sense.
Bill