I took risers off and risers and manifolds looked like new. Cleaned it up put back together with new gaskets. Replaced sea water impeller and housing which looked fine. Replaced spark plugs with oem and gapped to .035 per motor spec. Also took thermostat housing off and replaced thermostat which was bad and rusted.

Turned motor on and j initially idled fine and kept temp steady in middle at 180. Riser amd manifolds temps rose to 140 on starboard to 120 on port riser and about 10ndegrees less per side for manifolds.

In 3rd run noticed temp needs started climbing quickly. Shut it down. Risers were 170 degreed or so.

any places too look. I have a suspicion some gunk got loose and made it's way into cooling system. Thoughts appreciated

It should idle and cruise at about 165* on the water temp gauge. It might hit 175* on a hard run.
Did you use a Merc 160* Thermostat ?? Did you use a Merc OEM sea pump kit?? or off brand stuff???

Yes. Used merc seawater impeller and thermostat. Took apart cooling system in sections and flushed in both directions. Put back together and ran on muffs. This time risers and manifolds stayed under 100 degrees. Engine stayed around 160 except when bringing rpm up to 1500 and then back to idle. Jumped to 175 and them down again.

Ran about 10 minutes. Seemed to stay pretty cool. Good day there. Motor ran a bit rough. Just a little. I replaced plugs with oem and gapped to spec of .035. Didnt touch wires or cap. Also put 91 octane fuel in from gas station. Will explore further.

Probably bravoritus, check your water intake in the transom

Be careful running engine above idle on the hose. The sea water pump can typically move a greater volume of water than your hose can supply.

I have the double ears so water is coming in on both sides with decent pressure. Lots of water coming out of the exhaust. Wasn't planning on doing this a lot. I'll check that transom hose and try to figure out how to get to it to get it off and then back on. Any tips? Once the drive can lift itself (no trim pump installed yet) I'll try moving left or right to see if more space opens up.

Issue Solved. Will be checking Bravoitis at intake hose as well to maximize water intake. Noticed some corrosion nearby.

What was it?

Not exactly sure but I flushed all the hoses in sections and got some gunk out. Something must have gotten plugged up. That may have done it.

Old rubber from impeller still in system check & reflush both ways!