I had a similar problem on my old WD21, and it was a mixture of open stuck thermostat and tired gas pressure regulator. If the temperature remains low, the ECU thinks that the engine is still cold and enriches thoroughly. Replacing the thermostat seems like a good next step to me. Keep us posted, I'm interested!
You can post this as is on the forum. If you want a more technical or more formal version, I can offer you another one.