I finally had the time to work on my '98 today.  Leaving the carb in place I decided to just pull the bowl off and see what it might look like.  With the petcock in the ON position gas kept flowing.  To my mind that signifies a petcock issue.  A few weeks ago I checked the vacuum line (pulled it off the carb) to see if there was any gas in it.  It was fine, and no smell of gas in my oil.  
It appears from searching this forum that the way to go is a Raptor petcock, which I will go ahead and order.  I've never seen any gas puddles on the ground, so I am assuming that the valve in the carb is doing its job.
This leads me to a couple of questions:
1 - Could this constant flow of gas be contributing to the poor mileage I'm getting (35 - 39 mpg)?
2 - What are the chances of the top part of the carb being dirty (the bottom looked clean - no jet clogging)?

Does anyone know the Ron Ayers part numbers for a 150 main jet and a 55 pilot jet?  It's a 1998.