You have an air leak somewhere.

Make sure the bolts in the backplate are tight and make sure the carb is well seated against the o-ring. I usually press my thumb down hard on the carb while tightning the screw.

If that doesn't work, try the following to pin down where the air leak is. Do these ONE AT A TIME.

Take some vasoline and put it on pretty heavy all around the seam between the backplate and the crankcase of the engine. Start the engine, let it run about 30 - 40 seconds and see if it will shut down.

If it doesn't shut down repeat with the carb. Make sure the seam/gap between the carb and throat is completely covered - you may want to use a toothpick to get the areas that are hard to get to.

You only want to run the engine about 30 - 40 seconds before shutting it down because the heat generated by the engine can liquify the vasoline after much more time and make clean up more difficult.

If the vasoline does the trick, disassemble the part and clean the affected area with some alcohol. Then use some silicone sealer and put a bead of that on the mating surfaces, reassemble and let dry.

If it still won't shut down after using vasoline to seal any air leaks, the front bearing/seal is leaking OR there is an internal problem with the carb (barrel not fitting properly, barrel seals). You will need to call and return the engine.

Hope this helps.