Hi Plott
What I have also seen on these cube carburetors is the pump diaphragm not creating a perfect seal around the pulse port, so fuel gets sucked directly into the crankcase from the pump chamber. This will cause an over rich running condition with lots of smoke until the engine dies. I have taken to applying a trace of Loctite® or Permatex® flange sealant around the pulse port under the diaphragm being very careful to not let any sealant get close to the flapper valve of the pump diaphragm. Hope this gives you another avenue to explore. All the best,
Sky