Surprisingly to me, muffler feeds into 2/ 1/2 inch rubber tube which rises at least 2 feet then travels horizontal for 3 feet before dropping 2 feet to transom elbow outlet.
That loop is needed for the heel on sailing, but normally the exhaust is high enough above the water line to prevent back flow.
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Question: engine exhaust gas must pressurise mixed flow for it to travel that distance. ?
Answer is Yes. This causes the "spitting" or bursts of water, as your Genset engine Blows it out.
Perhaps your Muffler has crap it.
To hear the engine sounds, the muffler must not be filling with enough water.
Here is a picture of mine that was dunked for 10 minutes from Hurricane damage. I will get a new one by insurance.
So ignore the dirty state...
The Muffler is not a big thing, but hard to disconnect and clean.
You might Disconnect the hose at Riser and run a endoscope camera to see inside it, or disconnect that Loop and flush the Muffler clean with water hose.
Jim...