914UL carburettor gremlins

Thanks for all the input. The problem was finally identified about a year ago. Sadly, the aircraft has still been grounded due to other mechanical issues (not related to the engine) and my own lack of time to get them sorted.

It turned out that the float chamber was slightly loose due to the bowl nut/bolt not putting enough pressure on the chamber even when fully tightened. The mechanic discovered this by accident when he managed to twist the bowl after reassembling it. The whole thing was caused by the bottom gasket ("O-ring"?) having shrunk/become too thin due to old age...