From the same publication that EXDAC has provided is the following bulletin:
Article-Knowledge Base
Use of Carburetor Heat Control
Supersedes Service Instruction No. 1148C October 12, 2007 MODELS AFFECTED All Lycoming engines using float type carburetors. TIME OF COMPLIANCE During engine operation. Under certain moist atmospheric conditions (generally at a relative humidity of 50% or greater) and at temperatures of 20° to 90°F it is possible for ice to form in the induction system.