I experienced this with my 95 Four Winns 190 Horizon and Volvo 5.0 EFI at about 250 hours since new. With the outdrive trimmed up all the way, the exhaust bellows can be disconnected from the transom side to inspect inside the bottom end of the Y pipe. The flapper valves in my boat were rubber coated aluminum. One flapper came loose and partially blocked the exhaust in the transom area. This caused a significant power loss, and a loss of 12-15 mph top speed. We were able to fish a wire down through the y pipe and retrieve the flapper with the elbow and the bellows opened up. Some exhaust comes out under the transom shield around the transom anode at idle, but most of the exhaust goes through the prop hub at higher speeds. I also changed the anode and found a pin. The flapper installation includes the flapper, the pin, and the two rubber washers. A retrofit kit replaced the rubber washers with rubber flanged bushings.
The transom exhaust port is not as deep in the water as the prop hub, so less resistance for exhaust at idle. Hopefully, the hose between the exhaust elbow and the Y pipe can be positioned to cover the flapper pin holes or slots. You might also use tape and or silicone sealant on the holes according to the Service Bulletin. I experienced this also, but I was able to reposition the hose to stop the leak. Google "Volvo Penta flapper valve service bulletin" to find the Service Bulletin. Hope this helps.
The part that still puzzles me is Volvo/OMC using plastic parts in the exhaust system. These plastic parts and the rubber hoses are subjected to the water/exhaust, but generally are not compromised in normal use. Yet, if a plastic flapper comes loose and goes down to the bottom of the Y pipe further away from the combustion heat, many have said that the flapper melts and disappears.
