In my typical restoration projects, I have always used Easy Off oven cleaner to remove the carbon buildup in the exhaust housing. It can be harsh on the aluminum, so I typically let it soak for no more than 20 minutes or so. That’s usually enough to soften the carbon deposits that can then be removed with a good flush from the pressure washer. Here’s a pic of one such cleaning.
Bob
1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1955 Johnson QD-16
1957 Evinrude Fastwin 18
1957 Evinrude 3022
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20
1982 Evinrude 25hp
“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."