I have a 5h Briggs that goes on a tiller and it will not start. Its building up pressure in the bottom of the motor and blowing the tube off that runs up to the back side of the carb. Any ideas what is causing this??????

 

What engine are we dealing with? Briggs made several 5hp engines, some drastically different from others. Assuming this is a 130000 series, I'd say you need to pull the head and have a look at the piston (possibly hole in it) if it's building up that much pressure in the crankcase. If it's a newer OHV engine, it could also be a badly blown head gasket.