I would be suspecting a vacuum leak inside the motor if you can't find it by spraying a combustible material around the outside of the manifold. The intake face where it mates to the head could be a different angle, being gapped open on the inner side of the intake. The motor would thus be sucking non-combustible crankcase vapors into the ports. The same thing could happen even if the intake/head surfaces are perfectly parallel (not likely) if one or both of the intake gaskets are mis-aligned and not forming a perfect seal to the inside of the motor.