NOTE: I submitted this report to Fusioncopter Poland. This is a Hirth manufacturing defect, NOT FC's fault. If you notice weeping of burnt oil at the exhaust port, best to check and if necessary weld around the joint on the outside.Aircraft: Fusioncopter JK-2 NanoSerial no: FCG014Engine: Hirth F23Serial no:MO-01000570TIS: 15 hoursHistory: At about 5 hours TIS some exhaust weeping was noted around the LH exhaust port. It was assumed to be leaking at the flange gasket, so the exhaust flange was retorqued. The weeping continued, but as it was slight I continued operation intending to replace the gasket at next major service. During the last flight I noticed a change in engine sound with a slight reduction in rpm. On landing the engine sound immediately changed to unmuffled. On inspection the LH exhaust pipe had disconnected at the exhaust port flange.Analysis: The exhaust spigot had pulled out of the flange. The flange/spigot is assembled with the spigot inserted full depth in the flange, swaged(?) and welded around the spigot end. On close examination it appears only 1/3 to 1/2 of the weld fused with the spigot end. The muffler security was inspected prior to every flight with no evidence of excessive movement wrt the engine. The conclusion is the problem was due to inadequate welding spigot to flange. Corrective action: The flange and spigot were cleaned and dressed and assembled with the spigot reinserted full depth. The spigot was welded to the flange on the outside around the circumference. Not as tidy as the internal weld, but more substantial. Ground run- no leaks. Good to go!