I am not completely aufait with the carb heat installation on the A65, but to be able to answer you question I would like to know what aircraft it is fitted to and what RPM drop you do get when selecting carb heat. I did have a snag on an O200 where it had a massive carb heat drop and long recovery period, which was as a result of a cracked exhaust. I would also suggest looking at any seals on the carb heat butterfly so you rule out the possibility of the engine sucking warm air even when cold is selected. It is also of benefit to identify when you first noticed it and why you noticed it, had you or anyone carried out some maintenance prior to it happening. Hope this helps and I'll check back at a later to see if you need anymore info.