By "vent tube", are you referring to the road draft tube?

If air is exiting the road draft tube, that means air is getting past the piston and into the crankcase. You either have extremely worn or broken piston rings or a blown head gasket.

Air exiting the tailpipe is, of course, an exhaust valve.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)