Welch plugs are small, aluminum domed discs that are pressed into place over the low speed acceleration ports for access to these ports at a later date by the mechanic.
It's pretty rare to see an intake manifold cracked, but it does happen. Most often the replacement makes the machine run well again, as it no longer has an outside source of air.
It sounds like you have what Stihl calls a BIS carb, or Balanced Idle System. We call them a BS carb, that should give you a clue about them.
Once we determine the spark and compression are good, I will attemp to clean a carb one time. If it still will not pull fuel, a new carb is put on. A new carb is almost always a better fix. No comebacks and a better performing piece of equipment.
Needle limiter caps are a pita and we are always removing them so an adjustment can be made. They are a product of the clean air laws, as they will not let you richen the carb too much.
Robert, no offense, but if you don't even know what a venturi is, you really shouldn't be working on your carb. The 4137 series engine was a good one, we were still selling the FS-80 up until just last week when it became NLA.