I had a similar problem with a K and B 40 and a slinline pitts muffler. Not enough pressure , in my case anyway.
I could literaly take the high end needle all the way out and it would still run!
I closed off one muffler tube, and the problem was solved. Been runnin it like that for 15 years.
You only need an exit hole the size of the carb intake. the motor is nuthin but an airpump.
Perhaps you could try it at least, and see if it works for ya. Hope that helps.
d