525SC Carburated with crossovers, is there any need to have thermostats? if so why.

The reason I ask is I have had a problem out of nowhere. I have newly built motors. After about 15 hours and after boating for a few hours one day
my starboard motor after getting back on plane after a long channel ride started to get warm just at 200, when back to idle it started to drop.

We have replaced all of the somewhat antiquated cooling system with all new Merc stuff and hoses. The old sea water pumps did show wear but the impellers were intact. The motors have crossovers and the Stainless Marine T-Stat adaptors. I really don't know if it has T-Stats but I'm guessing yes that cruising is at 150 degrees, I'd like some other thoughts before taking it apart and possibly removing them.

Other than the temp rise it runs great. I really cant water test at the moment because we are still in a closed no wake scenario because of flooding. I was hoping to get this complete before reopening.

New exhaust? Could be a blockage anywhere. No matter how new impellers are they can't be ruled out either. If there are stats that one could be failing not letting it open all the way. Just a couple ideas.

I bought the boat in December all are very reputable builder, seller etc. much history on boat. I'm not sure if exhaust is new but looks it and operates as it should. The old water pumps were replaced and worn but both impellers were intact. We may already have this fixed, I was just thinking that since I can't boat and really not water test that I may check that as well. Because of it being a blower motor and with crossovers if it does have T-stats does it even need them and if I take them out am I causing another issue?

Does not have pressure gauges and it looks like all is equal now. I did not check that before we tore into the raw water pumps. Like I said it ran great for about 15 hours before I had any issue. We replaced hoses pump assemblies, made sure all hoses were clear. All headers seem equal temp. It may be okay now but trying to chase down everything since I can't boat anyway.

Still climbing temps under power, it immediately drops at idle, I'm thinking because of bypass holes in T-Stat. Changing thermostat and will drain down blocks to make sure they run clean. I will post results after.

Just a sticking T-Stat. Running great. Upgrading and replacing old raw water pumps and hoses although maybe not necessary didn't hurt.

Thanks for the input.