Leaky Anti-run-on valve?

["Steve B." <StevesTR@veroquest.com>]

Hi-
  I suspect that my Anti-run-on valve may be leaking. If I disconnect
the tube leading from the valve to the manifold and  cap the opening,
the car starts running better. Do these valves normally seal off
completely or is there a little bias leakage? Seems like any leakage
would be bad. Anyway, before I replace it at $50.00 I thought I'd ask.
 
Thanks,
Steve