S**t. Got that from a British sailor that was ripped off by poor Vetus quality once too many times.

The Vetus mufflers are vacuformed plastic and no where near as good as the Vernalift fiberglass ones.

If you have buildup all the way to the waterlift that engine is running either too cool or at too low RPMs.

Time to check the total exhaust system and you should run that engine a near full RPMs for at least 30 minutes a couple of times a season.