Blowback through carb

[Oxford Jack]

This sounds like a typical symptom of lean running, spitting back through the carb. I would suspect first an air leak, quite possibly from having a bend in the face flange, carb to manifold. Alternatively incorrect fuel level in the float chamber. The later best checked with an appropriate drill bit under the shut off mechanism. Also check the end of the butterfly spindle for a leak. A dob of grease on a suspect leak can be helpful.