I had an interesting discussion last night with a guy who use to fly ultralights. (He called after seeing my plane flying around saturday and wants to get into RC) He told me that my carb was too lean (I was thinking it was too rich) what he said is that the increased pressure in the cowl was making the engine run rich, and when the airspeed decreased to the point where the cowl pressure was below a certain level, then the carb pumped less fuel at idle (do to decreased pressure differential) and the engine would slow down.

I believe my engine is rich because if I let it run long enough 2-3 minutes at idle, it will load up and when I advance the throttle, it smokes bad and finally picks up RPM. I have been running lawnboy 32:1 and on my very next flight will be running Belray MC-1 at 50:1. I will see if this continues, but I know it happens. I also know that the 3 engines I have seen doesn't constitute all, but it is interesting some do it and some don't!