The Macs for a OS 46FX fits it. They do have a little more back pressure, therefore good fuel draw.

Could you just try setting the High needle correct and the low a little lean to see what your mid range does?

Another thing is that if the spring is missing or weak that fits behind the carb barrel, The barrel will flop in and out and make your mixture be all over the place?

The MC 55 uis supposed to be like the OS #3 and #8 but there may be some difference in yours?

Good luck,

Jim