I got a chance to do some more work on the 129 today. Several weeks ago I replaced a bad camshaft and installed some new rings while I had it apart. In hindsight, I should have left the old rings alone, for when I fired it up it smoked badly, even after an extended run, though it ran decently until the plug fouled.
After thinking about it for a couple of weeks, I decided to pull the motor and check the rings. I found one ring was installed upside down, and corrected the mistake. After I got it back together, it smoked a little less, but I was having problems with the carburetor "leaning out" after a few minutes of running. This weekend, I resolved to take care of the stalling issue. I pulled and cleaned the carburetor, and installed new fuel line and one of those tiny fuel filters with the brass element. Thankfully the engine ran pretty good, though it still smoked a good bit cold. The smoking seemed to mostly clear up after about 20 minutes of periodically running it hard uphill to put a load on it. I would have gone longer, but one of the tire chains started flapping, so I had to cut it short.
Here is a picture of the fuel filter setup, I installed the filter so the outside of the element was on the upstream side of the fuel flow, I am pretty sure I am right, but I want to make sure. Sorry about the picture quality, the florescent lights take forever to come to full brightness in a cold garage.