I have a 1999 Ranger 21. Bought it in 2004 with 1/2 hr on it, now has 3172 hrs. Put it to fultime use in early 07. Has been in the water with exception of 2 wks in 09 to paint the bottom and 2 days last wk to finally repaint. (barnacle and crap buildup is quite impressive. Bottom cleaning vs paint - great conversation. The original 21 will never hit10 mph in my experience but a clean bottom and a following sea will see 7.5. My experience of course.)
Mixing elbow carbon build-up problems tend to be caused from low rpm ie. 1200-1500. 3 simple sol'ns. -1. follow the book. After 1 hr of puttering, run at 2800-3000 rpm for 10 minutes, 2 pull the small elbow from the heat exchanger (it dumps the h2o in) and clean both, 3 remove the mixing elbow - the hose becomes well attached from the heat so jam a screwdriver in to separate, then clean-out the carbon. Don't waste your time being gentle (see the part about these boats can take more than the owner). After reinstalling, 1 do #1 or just run at a higher rpm.
Typically now run 2400rpm and have no problems. (2400rpm gives 2.8-3 hrs per gal at 6.6 or 6.8)
Note- my cast iron mixer has a silver plug on top. I assume the dis-similar metals applies here. No leaks at this point.)