Took the Donzi out for a good run yesterday, first long run since I've owned it & after a few repairs. I noticed that the engine temp only got up to about 150, & as soon as I came off plane, it would drop to 130. Right now the Gulf water temp is about 80, & I was running about 4000 rpms. I noticed the transom had a bit of black staining on it when I loaded it on the trailer. I'm thinking the engine should be running a bit warmer than 150, & the fact that it would drop to 130 so quickly....does this sound like it may not have a thermostat, or just a much cooler one than it needs? I'm also thinking that would answer why the black staining on the transom from running rich. What should this engine be running at normal operating temp?

That motor should have a 160* thermostat. Mine ran around 170* regardless of the water or air temp.

Sounds like its got a 140F t-stat in there to me. Replace with 160F one as Dave suggests.

Ditto.

Salt water tends to cystalize rather quickly at temps above 140 degrees. I know when my dad was cheif in the Navy, any equipment they used raw water cooling ran at 140 or less. I'm sure there's a slight safety factor at 140. My Donzi runs right at 140, with no t stat, which is exactly where it ran when it had a t stat.

Imho, oil temp is much more important.

I appreciate the input, that is what I was thinking, but it's always good to get opinions from folks with a bit more experience. I've dealt with automotive for a long time, & I'm cautious to immediately assume what works for cars will work in marine applications. Thanks again.

My only concern with the cooler temp is the fact that it is EFI. I know EFI likes warmer temps, & I was linking my rich condition to an engine that is not completely reaching it's normal operating range. That is my experience with automotive...I know, two different animals....

I think that's why you see modern Efi with closed cooling.

Good point...
Checked this evening, 140 t-stat was in it, I'll get a 160 tomorrow, I'm sure the computer will be much happier....LOL...
I'm also retired Navy 2001, from the Seabees, when did your dad retire? What was his rate?