Carb ice query

Hi all

Today I was ground running a 912ULS on a very cold and damp day in the UK.

The engine installation does not have any kind of carb heat for the body itself nor does it have any method of warming the inducted air (each carb is fitted with individual K&N air filters). From a cold start with the top engine cowl removed both carbs are very cold to the touch and condensation is seen to build on the outer surface of both carb bodies around the area of the venturi. As the engine starts to warm (oil temps rising over 25*C) the port side carb body remains dry after wiping its surface and feels warmer to the touch. The SB carb however, continues to form condensation on its surface after wiping dry and remains cold to the touch. Further when both carbs are viewed from the side the dry PT carb suffers minimal vibration whereas the SB carb suffers noticeable vibration. Both carbs are pneumatically balanced on a fine setting using a carb mate. 

Im assuming that the condensation is likely an indication of carb icing. The engine runs smooth at low rpms from 1450-1700rpm then runs slightly rougher than expected from 1700rpm to around 2700rpm. The engine begins to run smoother again from 2700rpm up to max rpm. I'm also therefore thinking that carb icing is taking affect on the early needle jet circuit. 

My question is why if all other factors seem equal between the two carbs should one carb experience carb icing on a ground run but not the other? The condensation and slightly rougher running in the low-mid range rpm persists on the SB carb even when operating temps have been achieved (oil to 50*C).

I have little knowledge of the internal condition of these carbs other than having replaced the floats and checked for normal operation of the float bracket and valve with the carb remaining in situ. I have also checked both carb sockets for signs of deterioration and have performed a pneumatic balance which balances well throughout the entire rev range. The engine apparently has logged around 300hrs. 

I'm just curious mainly so any comments would be welcome. 

I am hoping to ground run the engine again on a different day to see if the issue remains given different atmospheric conditions. I have previously run the engine during its initial carb balance and the slight rougher running was not noticed that time round. 

Cheers Shelley