I’ve used the Birel setup on several shifter karts and it works fine.
Pull your cable out of the sleeve and make sure it is not unraveling or kinked internally.
I assume that the clutch assembly in your engine was put together correctly and make sure that there are no grooves warn in the clutch basket. Pull the basket apart and start from there. Also, make sure that the rod is in correctly and there is a ball in there under the actuator hat. Double check that the arm next to the stator is assembled properly. (Hard to get wrong, but who knows). Also make sure that the lever is not overtightened on the steering pivot. I’ve used this assembly on my Birel, KGB, and Merlin and it should work fine. No need to spend dollars or get overly creative.