Well my problem is solved ! It turns out as suspected. The cowl caused the issue. It looks like there was a kind of 'vacuum' within the cowl on the carburetor's side (engine mounted sideways) so the engine started running 'rich' Solution was to simply (dremel) cut a few slots on the carb's side of the cowl allowing the engine to "breath". Never before had this kind of issue. I had engines overheating due to bad cooling inside a cowl several times before, but never this. You learn something new every day ;-)) even at 51.