One thing that you might check before doing anything like buying a new carb or whatever is that you might have an intake manifold leak. That will cause the motor to pop. The factory seals on the intake are not a very good design. Go find you an tube out of a 20" bike tire and cut 2 pieces about an inch wide. Install these pieces over the stock gaskets where the intake joins the heads. (it is a procedure that will probably make you cuss too!) Wrap them up with a little electrical tape and then put the clamps back on. Let me know if this solves your problem.