The Walbro WT-144 was the original carb for the Quadra 50 and 52. It is no longer made, however you can buy one here:
http://www.m-and-d.com/WL-WT-144.html (I bought one from here last August). I had similar problems with a very old Quadra 50 last summer. I replaced the carb, all the gaskets, bearings and seals (I knew there was an air leak somewhere, but I couldn't trace it down) so I replaced everything at once (mostly to save time) and that fixed it. I tried a Zama carb but I couldn't get the pulse port to line up perfectly, it was slightly off.
Grizak