Thank you for answering.
I tried to invert the polarity of autochocke, it didn't help. I guess the original polarity was ok.
I created a video with my problem though. This one below is a thorttle play the first 2-3 minutes after startup. Notice the fall of rpm each time I pull the throttle.
This other video is taken about 3 minutes later. You may notice the problem is not so evident as before.
Today was 63F (17 Celsius) and outside humidity is 28%. The problem is more serious in colder weather.
Any idea what is the problem?