Carb Icing

Hi all,

Would appreciate views on what I think can only be carb icing..

CS/FAR 23 certified aircraft, constant speed very light prop, carb heat through airbox/exhaust muffler.

What happens is that in ambient conditions of lower than 10 degrees C, high humidity: engine runs a bit rough after start-up. Once the engine is bit warmer I apply carb heat and the RPM initally drops a bit like it should, but if I wait it really rises, like from 2000 to 3500 RPM. When I disengage carb heat, engine runs smooth again at that lower 2000 RPM.

Again, only in these atmospheric conditions. 

I remember reading here that carb ice hardly ever happens at low RPM on the ground (I believe it was a comment by Roger, can't find it now).

The strange thing is that before this aircraft I owned the same brand, but ULM (LSA in US) with the same carb heat set up. But maybe I just did't notice it just doing a quick and not lengthy carb heat check. 

So my questions are:

1. Am I looking at carb icing here? If not, what else could this be?

2. How to explain that this did not seem to happen on my previous craft, other than I just never noticed.

I seriously doubt it's carb ice right at start up and at low rpms. I just doesn't have an opportunity to form that fast and with any amount that would make a difference.

When were the carbs last synced? Check your choke setup and movement.  Check for throttle free movement.  Air filters cleanliness. If they are K&N's are they over oiled? Hours on the engine and if it has quite a few and or the engine is years old carb overhaul.These are all simple quick checks.

Use gauges for the carb sync. You can diagnose with the gauges, but not with something like a CarbMate.