Here's a good reason to flush motors well.
I just got my boat a couple of years ago. Not too sure of the previous owner's routine but they did keep it moored for the summer months. On a trip that first year, about 5 -7 miles away from the marina, we could smell smoke and shut the engine down. Smoke was just billowing out when we stopped. Grabbed the fire extinguisher and popped the hood and the port bellows (rubber) was melting away with a gaping hole in it. Lucky there wasn't an actual fire. Made it home on the kicker, surprising how long it takes to get somewhere running the kicker at full speed, compared to how fast it takes to get somewhere while trolling.
The fault was the inside of the manifolds corroding and creating scale. The amount and size of the scaling was surprising. Some of the scaling came loose and plugged the holes in the riser, not allowing water to discharge with the exhaust. The exhaust was so hot and without the water discharging, it melted the bellows. The fix was to take off the manifolds and get them an acid wash at the rad shop. Did both sides and obviously replaced the port bellows. The biggest cost was for new gaskets in the risers. four of them at 40 bucks a pop.
Hooking up the muffs is one of the first things I do when I get home and am unloading the boat. Always bring it up to temp. I keep the muffs on board and if I'm able, will hook up at a marina and flush before leaving. Same with the wheels, I've already coughed up enough dough for brakes.