I've been having the same problem with my 2100. I finally rebuilt it and did everything step by step out of the book and it works better than ever. My issue was getting the choke just right and the float set correctly. But I'm sure you've done all that if you're this frustrated and you do seem to know what you're doing.
Instead of looking to PC, how about looking around your area for a good carb shop? I found a great one in my old hometown. They could tune my car like it was brand new. Ask the local mustang clubs where they go. There has to be a good carb shop in the KC area. PC is going to cost you ALOT more money than anything local.
The 4100 is one of the best carbs ever made. I wouldn't trade it for anything if I were you. But if you do want to get rid of it let me know
Good luck.
BTW...though I've had good service on getting carb parts from PC, they aren't always the most friendly on the phone.
